fix(linq2sql): remove unread _breakdown field from builder

Resolves SonarQube S4487 in LinqQueryBreakdownBuilder.cs.

The field was assigned in the constructor but never read. Grep across
the project confirmed no external references.

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

src/Strata.SqlTools.LinqToSql/Builders/LinqQueryBreakdownBuilder.cs

@@ -8,7 +8,6 @@ namespace Strata.SqlTools.Builders.LinqToSql;
 /// </summary>
 public class LinqQueryBreakdownBuilder
 {
-    private readonly LinqQueryBreakdown _breakdown;
     private readonly List<string> _selectColumns = new();
     private string? _fromTable;
     private string? _whereClause;
@@ -21,7 +20,6 @@ public class LinqQueryBreakdownBuilder
     /// </summary>
     public LinqQueryBreakdownBuilder()
     {
-        _breakdown = new LinqQueryBreakdown();
     }
 
     /// <summary>