refactor(dedup): cross-dialect QueryBreakdown + StatementParser helpers (Cluster E)

Two shared scaffolds for blocks Sonar flagged across the SqlServer,
Snowflake, and PostgreSQL dialects:

1. **`AppendToClause` on `SqlBreakdownBase`** — collapses the "if
   clause is empty set it, else append `{operation} {sql}`; then merge
   comment with same rule" pattern that was repeated three times in
   each of SqlServer/Snowflake `QueryBreakdown`. The matching
   `AddWhereExpression` / `AddHavingExpression` / `AddWhereClause(string)`
   sites in both files now delegate to a single `protected static`
   helper. Operates against `ISqlClause`, so it works for both the
   `WhereClause` and `HavingClause` properties.

2. **`HandleDoubleQuoteAsIdentifier` on `SqlServer.StatementParser`** —
   PostgreSQL and Snowflake both override SqlServer's
   `HandleDoubleQuote` (which produces a string-literal token) to
   instead produce a `ColumnIdentifier` token. The two overrides had
   identical 14-line bodies. The shared logic now lives once, and
   each dialect's override is a one-liner that calls the helper.

Deliberately *not* refactored in this commit:
- The CTE WITH-clause SQL generation in SqlServer/Snowflake QueryBreakdown
  (lines ~537-560 / ~579-601 Sonar flagged) — the surrounding logic
  differs enough between the two that an extraction would obscure
  rather than clarify.
- The PG/Snowflake QueryBreakdown constructor pair (lines 40-58 /
  43-61) — only ~10 lines × 2; extracting requires either a new
  shared helper for ~20 lines of savings or moving up the inheritance
  chain, neither pays for itself.

All 1180 tests stay green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Strata.SqlTools.PostgreSql/Statements/StatementParser.cs

@@ -84,23 +84,7 @@ public class StatementParser : SqlServerStatementParser
     /// <param name="position">Current position in the SQL string.</param>
     /// <returns>Token and new position after the token.</returns>
     protected override ((TokenType type, string value, int position) token, int newPosition) HandleDoubleQuote(string sql, int position)
-    {
-        // PostgreSQL: double-quote is an identifier (like [brackets] in T-SQL)
-        int start = position;
-        position++; // Skip opening quote
-        var identifier = new StringBuilder();
-        while (position < sql.Length && sql[position] != '"')
-        {
-            identifier.Append(sql[position]);
-            position++;
-        }
-        if (position < sql.Length)
-        {
-            position++; // Skip closing quote
-        }
-
-        return ((TokenType.ColumnIdentifier, identifier.ToString(), start), position);
-    }
+        => HandleDoubleQuoteAsIdentifier(sql, position);
 
     /// <summary>
     /// Post-processes extracted clauses to handle PostgreSQL-specific LIMIT and OFFSET clauses.

src/Strata.SqlTools.Snowflake/Breakdowns/QueryBreakdown.cs

@@ -231,28 +231,7 @@ public class QueryBreakdown : SqlServerQueryBreakdown
         var visitor = isMicrosoftSql
             ? (IVisitor<string>)new SqlServerCommandVisitor()
             : new CommandVisitor();
-        var sql = expression.Accept(visitor);
-
-        if (string.IsNullOrWhiteSpace(WhereClause.Clause))
-        {
-            WhereClause.Clause = sql;
-        }
-        else
-        {
-            WhereClause.Clause = $"{WhereClause.Clause} {operation} {sql}";
-        }
-
-        if (!string.IsNullOrWhiteSpace(comment))
-        {
-            if (string.IsNullOrWhiteSpace(WhereClause.Comment))
-            {
-                WhereClause.Comment = comment;
-            }
-            else
-            {
-                WhereClause.Comment = $"{WhereClause.Comment} {comment}";
-            }
-        }
+        AppendToClause(WhereClause, expression.Accept(visitor), operation, comment);
     }
 
     /// <summary>
@@ -297,29 +276,9 @@ public class QueryBreakdown : SqlServerQueryBreakdown
 
         // Extract comments from the incoming SQL
         var cleanSql = parser.ExtractSqlComments(sql, out var comments);
+        var commentText = comments.Count > 0 ? string.Join(" ", comments) : null;
 
-        if (string.IsNullOrWhiteSpace(WhereClause.Clause))
-        {
-            WhereClause.Clause = cleanSql.Trim();
-        }
-        else
-        {
-            WhereClause.Clause = $"{WhereClause.Clause} {operation} {cleanSql.Trim()}";
-        }
-
-        // Merge comments
-        if (comments.Count > 0)
-        {
-            var newComment = string.Join(" ", comments);
-            if (string.IsNullOrWhiteSpace(WhereClause.Comment))
-            {
-                WhereClause.Comment = newComment;
-            }
-            else
-            {
-                WhereClause.Comment = $"{WhereClause.Comment} {newComment}";
-            }
-        }
+        AppendToClause(WhereClause, cleanSql.Trim(), operation, commentText);
 
         // Extract and add parameters from the WHERE clause using appropriate parser
         ExtractAndAddParametersWithParser(cleanSql, parser);
@@ -476,28 +435,7 @@ public class QueryBreakdown : SqlServerQueryBreakdown
         var visitor = isMicrosoftSql
             ? (IVisitor<string>)new SqlServerCommandVisitor()
             : new CommandVisitor();
-        var sql = expression.Accept(visitor);
-
-        if (string.IsNullOrWhiteSpace(HavingClause.Clause))
-        {
-            HavingClause.Clause = sql;
-        }
-        else
-        {
-            HavingClause.Clause = $"{HavingClause.Clause} {operation} {sql}";
-        }
-
-        if (!string.IsNullOrWhiteSpace(comment))
-        {
-            if (string.IsNullOrWhiteSpace(HavingClause.Comment))
-            {
-                HavingClause.Comment = comment;
-            }
-            else
-            {
-                HavingClause.Comment = $"{HavingClause.Comment} {comment}";
-            }
-        }
+        AppendToClause(HavingClause, expression.Accept(visitor), operation, comment);
     }
 
     /// <summary>

src/Strata.SqlTools.Snowflake/Statements/StatementParser.cs

@@ -91,23 +91,7 @@ public class StatementParser : SqlServerStatementParser
     /// <param name="position">Current position in the SQL string.</param>
     /// <returns>Token and new position after the token.</returns>
     protected override ((TokenType type, string value, int position) token, int newPosition) HandleDoubleQuote(string sql, int position)
-    {
-        // Snowflake: double-quote is an identifier (like [brackets])
-        int start = position;
-        position++; // Skip opening quote
-        var identifier = new StringBuilder();
-        while (position < sql.Length && sql[position] != '"')
-        {
-            identifier.Append(sql[position]);
-            position++;
-        }
-        if (position < sql.Length)
-        {
-            position++; // Skip closing quote
-        }
-
-        return ((TokenType.ColumnIdentifier, identifier.ToString(), start), position);
-    }
+        => HandleDoubleQuoteAsIdentifier(sql, position);
 
     /// <summary>
     /// Post-processes extracted clauses to handle Snowflake-specific LIMIT clause.

src/Strata.SqlTools.SqlBreakdown/Classes/SqlBreakdownBase.cs

@@ -53,6 +53,42 @@ public abstract class SqlBreakdownBase : ISqlBreakdown
     /// <returns>The SQL query string.</returns>
     protected abstract string GetSqlBreakdown();
 
+    /// <summary>
+    /// Appends a SQL fragment to an <see cref="ISqlClause"/> with the given logical operation
+    /// (e.g. <c>"and"</c> / <c>"or"</c>), and optionally merges an associated comment.
+    /// If the target clause is empty, the fragment is set as the clause; otherwise it is
+    /// joined with the operation.
+    /// </summary>
+    /// <param name="clause">The clause being built up (e.g. <c>WhereClause</c>, <c>HavingClause</c>).</param>
+    /// <param name="sql">The already-generated SQL fragment to append.</param>
+    /// <param name="operation">The logical operation used to join with the existing content.</param>
+    /// <param name="comment">Optional comment to merge into <c>clause.Comment</c>.</param>
+    protected static void AppendToClause(ISqlClause clause, string sql, string operation, string? comment)
+    {
+        if (string.IsNullOrWhiteSpace(clause.Clause))
+        {
+            clause.Clause = sql;
+        }
+        else
+        {
+            clause.Clause = $"{clause.Clause} {operation} {sql}";
+        }
+
+        if (string.IsNullOrWhiteSpace(comment))
+        {
+            return;
+        }
+
+        if (string.IsNullOrWhiteSpace(clause.Comment))
+        {
+            clause.Comment = comment;
+        }
+        else
+        {
+            clause.Comment = $"{clause.Comment} {comment}";
+        }
+    }
+
     /// <summary>
     /// Gets a LINQ to SQL query of the specified type based on this breakdown.
     /// </summary>

src/Strata.SqlTools.SqlServer/Breakdowns/QueryBreakdown.cs

@@ -822,29 +822,9 @@ public class QueryBreakdown : SqlBreakdownBase, IQueryBreakdown
 
         // Extract comments from the incoming SQL
         var cleanSql = Parser.ExtractSqlComments(sql, out var comments);
+        var commentText = comments.Count > 0 ? string.Join(" ", comments) : null;
 
-        if (string.IsNullOrWhiteSpace(WhereClause.Clause))
-        {
-            WhereClause.Clause = cleanSql.Trim();
-        }
-        else
-        {
-            WhereClause.Clause = $"{WhereClause.Clause} {operation} {cleanSql.Trim()}";
-        }
-
-        // Merge comments
-        if (comments.Count > 0)
-        {
-            var newComment = string.Join(" ", comments);
-            if (string.IsNullOrWhiteSpace(WhereClause.Comment))
-            {
-                WhereClause.Comment = newComment;
-            }
-            else
-            {
-                WhereClause.Comment = $"{WhereClause.Comment} {newComment}";
-            }
-        }
+        AppendToClause(WhereClause, cleanSql.Trim(), operation, commentText);
 
         // Extract and add parameters from the WHERE clause
         ExtractAndAddParameters(cleanSql);
@@ -901,28 +881,7 @@ public class QueryBreakdown : SqlBreakdownBase, IQueryBreakdown
         }
 
         var visitor = new CommandVisitor();
-        var sql = expression.Accept(visitor);
-
-        if (string.IsNullOrWhiteSpace(WhereClause.Clause))
-        {
-            WhereClause.Clause = sql;
-        }
-        else
-        {
-            WhereClause.Clause = $"{WhereClause.Clause} {operation} {sql}";
-        }
-
-        if (!string.IsNullOrWhiteSpace(comment))
-        {
-            if (string.IsNullOrWhiteSpace(WhereClause.Comment))
-            {
-                WhereClause.Comment = comment;
-            }
-            else
-            {
-                WhereClause.Comment = $"{WhereClause.Comment} {comment}";
-            }
-        }
+        AppendToClause(WhereClause, expression.Accept(visitor), operation, comment);
     }
 
     /// <summary>
@@ -1013,28 +972,7 @@ public class QueryBreakdown : SqlBreakdownBase, IQueryBreakdown
         }
 
         var visitor = new CommandVisitor();
-        var sql = expression.Accept(visitor);
-
-        if (string.IsNullOrWhiteSpace(HavingClause.Clause))
-        {
-            HavingClause.Clause = sql;
-        }
-        else
-        {
-            HavingClause.Clause = $"{HavingClause.Clause} {operation} {sql}";
-        }
-
-        if (!string.IsNullOrWhiteSpace(comment))
-        {
-            if (string.IsNullOrWhiteSpace(HavingClause.Comment))
-            {
-                HavingClause.Comment = comment;
-            }
-            else
-            {
-                HavingClause.Comment = $"{HavingClause.Comment} {comment}";
-            }
-        }
+        AppendToClause(HavingClause, expression.Accept(visitor), operation, comment);
     }
 
     /// <summary>

src/Strata.SqlTools.SqlServer/Statements/StatementParser.cs

@@ -649,6 +649,32 @@ public class StatementParser
         return ((TokenType.String, str.ToString(), start), position);
     }
 
+    /// <summary>
+    /// Helper used by dialects (PostgreSQL, Snowflake) where double-quoted text is an
+    /// identifier rather than a string literal. Reads from the opening quote at
+    /// <paramref name="position"/> and returns the inner text as a
+    /// <see cref="TokenType.ColumnIdentifier"/> token.
+    /// </summary>
+    /// <param name="sql">The SQL statement being tokenized.</param>
+    /// <param name="position">The current position in the SQL string (must point at the opening <c>"</c>).</param>
+    /// <returns>The identifier token and the new position past the closing <c>"</c>.</returns>
+    protected static ((TokenType type, string value, int position) token, int newPosition) HandleDoubleQuoteAsIdentifier(string sql, int position)
+    {
+        int start = position;
+        position++; // Skip opening quote
+        var identifier = new StringBuilder();
+        while (position < sql.Length && sql[position] != '"')
+        {
+            identifier.Append(sql[position]);
+            position++;
+        }
+        if (position < sql.Length)
+        {
+            position++; // Skip closing quote
+        }
+        return ((TokenType.ColumnIdentifier, identifier.ToString(), start), position);
+    }
+
     /// <summary>
     /// Processes a list of tokens to find SQL keywords at the top level (outside parentheses).
     /// </summary>