chore(sonar)!: mark public Markdown generator methods static (CA1822)

Three public methods on the SqlServer-namespaced Markdown generators no
longer touch instance state and now carry the `static` keyword:

- `Markdown.SqlServer.QueryBreakdownGenerator.GenerateMermaidDiagram(QueryBreakdown, string?)`
- `Markdown.SqlServer.SqlStatementGenerator.GenerateSequenceDiagram(ISqlBreakdown, string?)`
- `Markdown.SqlServer.SqlStatementGenerator.GenerateEntityRelationshipDiagram(IEnumerable<string>, string?)`

Plus one private bonus the analyzer caught on the same pass:
- `LinqExpressionVisitor.ExtractSelectExpression` → static (non-breaking).

Internal callers in the Snowflake/LinqToSql/PostgreSql wrapper classes
and in the test fixtures are updated to the type-name form
(`SqlServer.SqlStatementGenerator.GenerateSequenceDiagram(...)`).
The wrappers retain their `_baseGenerator` field for now even though it
is no longer used — that S4487 / unused-field cleanup is its own commit.

BREAKING CHANGE: External NuGet consumers calling
`generatorInstance.GenerateMermaidDiagram(...)`,
`generatorInstance.GenerateSequenceDiagram(...)`, or
`generatorInstance.GenerateEntityRelationshipDiagram(...)` on the
SqlServer-namespaced generators must switch to type-name form, e.g.
`Markdown.SqlServer.SqlStatementGenerator.GenerateSequenceDiagram(...)`.
Calls through the Snowflake / LinqToSql / PostgreSql wrapper classes are
unaffected at the call site.

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

tests/Strata.SqlTools.Markdown.Tests/SqlServer/SqlStatementGeneratorTests.cs

@@ -21,7 +21,7 @@ public class SqlStatementGeneratorTests
         var query = new QueryBreakdown("*", "Users");
 
         // Act
-        var result = _generator.GenerateSequenceDiagram(query);
+        var result = SqlStatementGenerator.GenerateSequenceDiagram(query);
 
         // Assert
         Assert.That(result, Does.Contain("```mermaid"));
@@ -39,7 +39,7 @@ public class SqlStatementGeneratorTests
         query.SetupClauses.Add("CREATE TEMP TABLE #Temp (ID INT)");
 
         // Act
-        var result = _generator.GenerateSequenceDiagram(query);
+        var result = SqlStatementGenerator.GenerateSequenceDiagram(query);
 
         // Assert
         Assert.That(result, Does.Contain("Setup Clauses"));
@@ -55,7 +55,7 @@ public class SqlStatementGeneratorTests
         query.FinishClauses.Add("DROP TABLE #Temp");
 
         // Act
-        var result = _generator.GenerateSequenceDiagram(query);
+        var result = SqlStatementGenerator.GenerateSequenceDiagram(query);
 
         // Assert
         Assert.That(result, Does.Contain("Finish Clauses"));
@@ -69,7 +69,7 @@ public class SqlStatementGeneratorTests
         var query = new QueryBreakdown("*", "Users");
 
         // Act
-        var result = _generator.GenerateSequenceDiagram(query, "Query Execution");
+        var result = SqlStatementGenerator.GenerateSequenceDiagram(query, "Query Execution");
 
         // Assert
         Assert.That(result, Does.StartWith("### Query Execution"));
@@ -82,7 +82,7 @@ public class SqlStatementGeneratorTests
         var tables = new[] { "Users", "Orders", "Products" };
 
         // Act
-        var result = _generator.GenerateEntityRelationshipDiagram(tables);
+        var result = SqlStatementGenerator.GenerateEntityRelationshipDiagram(tables);
 
         // Assert
         Assert.That(result, Does.Contain("```mermaid"));
@@ -100,7 +100,7 @@ public class SqlStatementGeneratorTests
         var tables = new[] { "Users", "Orders" };
 
         // Act
-        var result = _generator.GenerateEntityRelationshipDiagram(tables, "Database Schema");
+        var result = SqlStatementGenerator.GenerateEntityRelationshipDiagram(tables, "Database Schema");
 
         // Assert
         Assert.That(result, Does.StartWith("### Database Schema"));
@@ -113,7 +113,7 @@ public class SqlStatementGeneratorTests
         var tables = new[] { "[dbo].[Users]", "schema.Orders" };
 
         // Act
-        var result = _generator.GenerateEntityRelationshipDiagram(tables);
+        var result = SqlStatementGenerator.GenerateEntityRelationshipDiagram(tables);
 
         // Assert
         Assert.That(result, Does.Contain("dbo_Users"));