Fix C# generated attribute back compatibility

[live1206]

Summary

• add an opt-in, write-time TypeProvider hook for emitting selected attributes from the previous GA contract without placing them in the normal attribute/visitor/reference-map pipeline
• preserve prior ModelReaderWriterBuildableAttribute entries only when their target and every enclosing type and generic argument remain emitted or otherwise resolvable, while deduplicating entries still generated today
• exclude unrelated attributes such as EditorBrowsableAttribute, removed targets, and any target component marked error-obsolete (Obsolete(..., true))
• recursively collect, deduplicate, and reapply every obsolete and experimental diagnostic suppression required by the target, its declaring types, and its generic arguments
• correctly resolve and render arbitrarily nested target types, including constructed generic declaring types

Validation

• npm run build
• 1,719 Microsoft.TypeSpec.Generator tests
• 1,514 Microsoft.TypeSpec.Generator.ClientModel tests
• npm run cop
• full C# test-library regeneration
• pnpm format:check
• type-aware oxlint

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11364

commit: 28da475

[github-actions]

No changes needing a change description found.

[jorgerangel-msft]

fyi, I was tackling this in #11237.

[JoshLove-msft]

The nested generic type rendering fixes in CSharpType/CodeWriter make sense, but I don't think this compatibility fix needs three additional overridable TypeProvider hooks (MergeLastContractAttributesForWrite, ShouldPreserveLastContractAttributeForWrite, and BuildLastContractAttributeForWrite).

ModelReaderWriterContextDefinition is currently the only consumer, and its override effectively merges through base, filters the result, and then merges again. Could the preservation/filtering/suppression logic remain private in ModelReaderWriterContextDefinition, with the existing BuildAttributesForWrite override as the sole extension point? Unless we expect other providers to need exactly this write-time last-contract attribute pipeline, the new protected-internal API surface seems broader and more specialized than the fix requires.

--generated by Copilot