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/QueryBreakdownGeneratorTests.cs

@@ -22,7 +22,7 @@ public class QueryBreakdownGeneratorTests
         var query = new QueryBreakdown("ID, Name", "Users");
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query);
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query);
 
         // Assert
         Assert.That(result, Does.Contain("```mermaid"));
@@ -46,7 +46,7 @@ public class QueryBreakdownGeneratorTests
         query.OrderByClause.Clause = "OrderCount DESC";
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query, "User Orders Query");
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query, "User Orders Query");
 
         // Assert
         Assert.That(result, Does.Contain("### User Orders Query"));
@@ -65,7 +65,7 @@ public class QueryBreakdownGeneratorTests
         var query = new QueryBreakdown("*", "Products", "Price > 100");
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query);
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query);
 
         // Assert
         Assert.That(result, Does.Contain("WHERE"));
@@ -86,7 +86,7 @@ public class QueryBreakdownGeneratorTests
         var query = QueryBreakdown.Parse(sql);
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query);
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query);
 
         // Assert
         Assert.That(result, Is.Not.Null);
@@ -102,7 +102,7 @@ public class QueryBreakdownGeneratorTests
         var query = new QueryBreakdown("*", "Users");
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query, "Test Query");
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query, "Test Query");
 
         // Assert
         Assert.That(result, Does.StartWith("### Test Query"));
@@ -115,7 +115,7 @@ public class QueryBreakdownGeneratorTests
         var query = new QueryBreakdown("*", "Users");
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query);
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query);
 
         // Assert
         Assert.That(result, Does.Not.StartWith("###"));
@@ -130,7 +130,7 @@ public class QueryBreakdownGeneratorTests
         var query = new QueryBreakdown(longSelect, "LargeTable");
 
         // Act
-        var result = _generator.GenerateMermaidDiagram(query);
+        var result = QueryBreakdownGenerator.GenerateMermaidDiagram(query);
 
         // Assert
         Assert.That(result, Does.Contain("..."));