fix(rules): suppress S3875 on intentional DSL operator==

Resolves SonarQube S3875 (BLOCKER) in Expression.Operators.cs.

The operator== returns a Comparison expression (DSL semantics), not a
bool. The existing CS0660/CS0661 pragma already documents this design;
the new SuppressMessage attribute makes the Sonar analyzer agree.

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

src/Strata.SqlTools.Rules/Rule/Expression/Expression.Operators.cs

@@ -1,9 +1,12 @@
+using System.Diagnostics.CodeAnalysis;
+
 namespace Strata.SqlTools.Rules.Rule.Expression;
 
 /// <summary>
 /// Provides implicit conversion operators and comparison operators for rule expressions.
 /// </summary>
 #pragma warning disable CS0660, CS0661
+[SuppressMessage("Major Code Smell", "S3875:\"operator==\" should not be overloaded on reference types", Justification = "Intentional DSL syntax: `expr1 == expr2` constructs a Comparison rule expression, not a bool. The existing CS0660/CS0661 pragma documents the deliberate omission of Equals/GetHashCode for the same reason.")]
 public partial class Expression
 #pragma warning restore CS0660, CS0661
 {