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>

src/Strata.SqlTools.Markdown/LinqToSql/QueryBreakdownGenerator.cs

@@ -28,7 +28,7 @@ public class QueryBreakdownGenerator
     {
         // Since LinqQueryBreakdown inherits from SqlServer.QueryBreakdown,
         // we can use the base generator which works with the shared properties
-        return _baseGenerator.GenerateMermaidDiagram(queryBreakdown, title);
+        return SqlServer.QueryBreakdownGenerator.GenerateMermaidDiagram(queryBreakdown, title);
     }
 
     /// <summary>