chore(sonar): mark private helpers static (CA1822)

Two private methods that don't touch instance state get the `static`
keyword:

- `LinqExpressionVisitor.ExtractMemberName`
- `Markdown.Expressions.ExpressionGenerator.GenerateMermaidDiagram(Expression)`

Both have only intra-class callers, so this is a non-breaking change —
the call sites continue to work unchanged under C# method-resolution.

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

src/Strata.SqlTools.LinqToSql/Visitors/LinqExpressionVisitor.cs

@@ -303,7 +303,7 @@ public class LinqExpressionVisitor : ExpressionVisitor
         return string.IsNullOrEmpty(name) ? "*" : name;
     }
 
-    private string ExtractMemberName(Expression expression)
+    private static string ExtractMemberName(Expression expression)
     {
         if (expression is MemberExpression member)
         {

src/Strata.SqlTools.Markdown/Expressions/ExpressionGenerator.cs

@@ -245,7 +245,7 @@ public class ExpressionGenerator : IVisitor<string>
     /// <summary>
     /// Generates a Mermaid tree diagram from an Expression tree.
     /// </summary>
-    private string GenerateMermaidDiagram(Expression expression)
+    private static string GenerateMermaidDiagram(Expression expression)
     {
         var sb = new StringBuilder();
         sb.AppendLine("```mermaid");