Restore missing last-contract buildable attributes on ModelReaderWriterContextDefinition

[Copilot]

ModelReaderWriterContextDefinition rebuilt its [ModelReaderWriterBuildable(...)] attributes purely from the current generation, so a type dropped from the spec silently lost its buildable entry — a source-breaking change for consumers that rely on the context to build that type. Attributes are now reconciled against the last contract.

Changes

• ModelReaderWriterContextDefinition.BuildAttributes() — after collecting generated + customized entries, restore any ModelReaderWriterBuildableAttribute present on LastContractView that the current generation no longer produces.

  • Last-contract attributes are symbol-based (IsFrameworkType == false), so they're matched by Type.FullyQualifiedName rather than Type.FrameworkType.
  • De-duplication is by the target type's simple name across generated, customized, and last-contract entries; last-contract targets can render with an empty namespace (global::.Foo) when the referenced type isn't defined in that compilation.
  • Restored entries are keyed by type identity so they sort consistently with generated ones. IsBuildableAttribute already recognizes the restored attributes, so BuildAttributesForWrite does not re-emit them.

• Tests — LastContractBuildableAttributesAreRestoredWhenMissing (removed type restored, present type not duplicated) and BuildAttributesForBackCompatibilityDeduplicatesAcrossGeneratedCustomAndLastContractBuildableAttributes (dedupe across all three sources), plus supporting SampleContext.cs TestData.

Given a last contract that shipped RegularModel + RemovedModel but a generation that only produces RegularModel, the emitted context is:

[global::System.ClientModel.Primitives.ModelReaderWriterBuildableAttribute(typeof(global::Sample.Models.RegularModel))]
[global::System.ClientModel.Primitives.ModelReaderWriterBuildableAttribute(typeof(global::Sample.Models.RemovedModel))]
public partial class SampleContext : global::System.ClientModel.Primitives.ModelReaderWriterContext
{
}

• Fixes Add Back Compat Attribute Processing for ModelReaderWriterContextDefinition #11378

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[jorgerangel-msft]

@copilot add at least 1 more unit test that covers a case where there are attributes in custom code, back compat, and in the generated layer

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commit: 630e1e8

[Copilot]

@copilot add at least 1 more unit test that covers a case where there are attributes in custom code, back compat, and in the generated layer

Added BuildAttributesForBackCompatibilityIncludesGeneratedCustomAndRestoredLastContractBuildableAttributes, which covers generated (GeneratedModelA/GeneratedModelB), customized (CustomModel), and last-contract-only (RemovedModelA/RemovedModelB) buildable attributes together, validated against a TestData expected file.

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