[http-client-csharp] Consolidate duplicate MethodSignature comparison logic

[JonathanCrd]

Fixes #9658

This PR removes duplicated method signature comparison logic by using a consolidated MethodSignatureEqualityComparer

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Changes

🔧 Created Unified Comparer

• Added MethodSignatureBaseEqualityComparer to MethodSignatureBase class
• Consolidated all comparison logic including special operator handling (explicit vs implicit)
• Exposed via public static readonly IEqualityComparer<MethodSignatureBase> SignatureComparer

🧹 Eliminated Duplication in TypeProvider

• Replaced ~40 lines of custom IsMatch() logic with single line delegation to base comparer
• Removed duplicate helper methods (IsNameMatch, GetFullMethodName)

🚀 Simplified MethodSignature Comparer

• Removed redundant MethodSignatureEqualityComparer wrapper class
• Changed to clean property: MethodSignatureComparer => SignatureComparer

📝 Updated References

• Updated 4 references in ClientProvider.cs and ModelFactoryProvider.cs to maintain existing API

Benefits

✅ Single source of truth - All method signature equality logic unified
✅ Extended to constructors - Base comparer now works for both methods and constructors
✅ Eliminated ~50 lines of duplicate code
✅ Backward compatible - No breaking changes to existing APIs
✅ All tests pass - Verified with full test suite (3,235 tests, 0 failures)

Testing

• All existing tests pass
• Constructor signature matching scenarios validated
• Method signature comparison behavior unchanged

[github-actions]

No changes needing a change description found.

[jorgerangel-msft]

Can we please add a few unit tests exercising the new comparer?

[JoshLove-msft]

Can we verify the fix in Search before merging?