Add Support for More Back Compat Model Ctor Scenarios

[Jorge Rangel (jorgerangel-msft)]

This PR introduces more covered scenarios for the back compat support for model ctors.

fixes: #11588

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Pull request overview

This PR expands the C# generator’s back-compat handling for model constructors (and a related synthesized-parameter method scenario) so that previously-published parameter names can be restored when matching last-contract signatures by parameter types, including swap/rotation cases and certain constructor-restore paths.

Changes:

• Adds constructor-parameter name restoration by last-contract signature matching (including swap/rotation and casing scenarios).
• Allows restoring certain previously-published constructors as standalone constructors when no chaining target exists (or when a required property can’t be assigned).
• Adds/updates unit tests and last-contract/expected-output fixtures to cover the added scenarios.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/BackCompatHelper.cs	Enables positional fallback for synthesized parameters even when ParameterProvider.Name differs from InputParameter.Name.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs	Implements ctor parameter-name restoration by signature and extends constructor restoration to standalone scenarios.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TypeProviderTests.cs	Adds a regression test for synthesized parameter name restoration via signature match.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityRestoresSynthesizedParameterNameBySignatureMatch/TestClient.cs	Adds last-contract input fixture for the synthesized-parameter test.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityRestoresSynthesizedParameterNameBySignatureMatch.cs	Adds expected generated output fixture for the synthesized-parameter test.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/ModelProviderTests.cs	Adds ctor back-compat tests for rename/swap/rotation/casing and standalone constructor restore scenarios.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterSwapRestoredBySignatureMatch/MockInputModel.cs	Adds last-contract input fixture for swapped ctor parameter names.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterSwapRestoredBySignatureMatch.cs	Adds expected output fixture for swapped ctor parameter names.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterRenameRestoredBySignatureMatch/MockInputModel.cs	Adds last-contract input fixture for renamed ctor parameter names.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterRenameRestoredBySignatureMatch.cs	Adds expected output fixture for renamed ctor parameter names.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterCasingAndRotationRestoredBySignatureMatch/Models.cs	Adds last-contract input fixtures for casing-only rename and 3-param rotation.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterCasingAndRotationRestoredBySignatureMatch(Casing).cs	Adds expected output fixture for casing-only ctor parameter restore.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ConstructorParameterCasingAndRotationRestoredBySignatureMatch(Rotation).cs	Adds expected output fixture for rotated ctor parameter restore.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_StandaloneConstructorRestoredWhenNoChainTarget/MockInputModel.cs	Adds last-contract input fixture for standalone ctor restoration when no chaining target exists.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_StandaloneConstructorRestoredWhenNoChainTarget.cs	Adds expected output fixture for standalone ctor restoration when no chaining target exists.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_StandaloneConstructorRestoredWhenRequiredPropertyUnassigned/MockInputModel.cs	Adds last-contract input fixture for restoring a ctor that can’t assign a newly-required property.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_StandaloneConstructorRestoredWhenRequiredPropertyUnassigned.cs	Adds expected output fixture for restoring a ctor that can’t assign a newly-required property.

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