Support back-compat model constructors in the C# generator

[Copilot]

When a previously-required model property is relaxed to optional, the generator drops the corresponding parameter from the model's initialization constructor — a source-breaking change for callers that construct the model positionally (issue #11460). This restores the previously-published public constructor via the LastContractView back-compat mechanism.

Changes

• ModelProvider.BuildConstructorsForBackCompatibility (new override) — For each public constructor in LastContractView that has no current equivalent, reconstructs it as a public overload that chains to the closest current public constructor (: this(...)) and assigns the extra properties in its body.
  • Chain target is the current public constructor whose parameters form an in-order subsequence of the previous ones (closest match preferred).
  • Each dropped parameter must map to a settable property with an unchanged type — matched by property name, or by OriginalName for codegen renames. If any dropped parameter cannot be mapped, restoration is skipped.
  • Kept parameters forward to the chained constructor (validation stripped, since the target validates); non-nullable reference-type extras reinstate their AssertNotNull check.
• BackCompatibilityChangeCategory — Adds ConstructorAddedFromLastContract and ConstructorAddedFromLastContractSkipped for emitter diagnostics.

Example

Given a model where resources was required and is now optional, the restored overload:

public MockInputModel(string name, string resources) : this(name)
{
    Argument.AssertNotNull(resources, nameof(resources));
    Resources = resources;
}

Tests

Added unit tests covering the restore case (required→optional), and the negative cases where the property was removed entirely or no last contract exists.

• Fixes LastContractView - Consider Supporting Back Compat Model Ctors #11460

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[jorgerangel-msft]

@copilot please add robust unit tests to test our changes. We should use TestData to validate the changes, and follow the existing pattern for validating back compat scenarios

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commit: 93bfb97

[Copilot]

@copilot please add robust unit tests to test our changes. We should use TestData to validate the changes, and follow the existing pattern for validating back compat scenarios

Added robust TestData-based tests in c2f7d78: BackCompat_RequiredToOptionalConstructorIsRestored and BackCompat_OptionalValueTypeConstructorParameterIsRestored now validate the full generated model (restored constructor + chaining + assignment) against expected .cs snapshots, and a new BackCompat_RenamedPropertyConstructorIsRestored test covers a [CodeGenMember] rename to exercise the CanonicalView.Properties/OriginalName matching, alongside the existing negative cases.

[jorgerangel-msft]

@copilot I"d like to see some unit tests where there are ctors in the back compat view, but also the ctor is replaced with custom code.

[Copilot]

@copilot I"d like to see some unit tests where there are ctors in the back compat view, but also the ctor is replaced with custom code.

Added in 255c02e — the new BackCompat_ConstructorNotRestoredWhenReplacedByCustomCode test provides the (name, resources) constructor in the last-contract view but also replaces it with a custom-code partial. The existence check now also scans CanonicalView.Constructors (which includes custom code), so the generator no longer emits a colliding overload. The test asserts the custom ctor is present in the canonical view and that no generated overload is added.

[Copilot]

@copilot lets update https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/generator/docs/backward-compatibility.md#model-constructors to include a sub-section explaining the new...

Documented the required-to-optional model-property scenario in 88060f1a, including restoration behavior, example output, eligibility constraints, custom constructors, renames, and ApiCompat baseline handling.