fix: relax back compat param name matching

[jorgerangel-msft]

Relaxes the back compat restoration of parameter names in methods. Previously, we only match a param to a previous signature's param if the current param's wire name matched the previous. Though there are still cases where the wire name changes and we would want the public surface of these methods to still not be unchanged from previous.

fixes: #11500

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commit: 55825aa

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Pull request overview

This PR adjusts the C# generator’s back-compat parameter name restoration to allow a signature-based (positional) fallback when the previously-published parameter name can’t be discovered via the spec/original name, and adds coverage to validate the new behavior.

Changes:

• Update BackCompatHelper.RestorePreviousParameterNames to fall back to matching last-contract methods by signature (ignoring parameter names) and restoring parameter names positionally when name-based lookup fails.
• Refine existing test coverage to ensure positional fallback does not occur when there is no matching last-contract signature.
• Add a new test + test data to validate restoring a renamed parameter purely via signature match.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TypeProviderTests.cs	Updates an existing scenario and adds a new unit test covering signature-based parameter-name restore.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityRestoresRenamedParameterBySignatureMatch/TestClient.cs	Adds last-contract input type for the new signature-match restore scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityRestoresRenamedParameterBySignatureMatch.cs	Adds expected generated output asserting the restored parameter name.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityKeepsUnpublishedParameterName/TestClient.cs	Adjusts the last-contract shape to ensure no signature match exists for the “keep name” test.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/BackCompatHelper.cs	Implements the relaxed matching logic to restore parameter names by signature when needed.

Comments suppressed due to low confidence (1)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/BackCompatHelper.cs:188

• This comment is now misleading: the positional fallback runs not only for synthesized parameters, but also when we can’t map by OriginalName (e.g., when a different generator renamed the parameter). Update the comment to match the behavior.

                    // Fall back to a positional match for synthesized parameters