Centralize type attribute back-compat merging in C# generator

[live1206]

Background

PR #10976 currently merges generated, last-contract, and custom-code type attributes locally in CanonicalTypeProvider.BuildAttributes(). This keeps the provider reference-map/pre-write internalization work scoped, but the back-compat behavior would be clearer and more consistent if it followed the existing centralized TypeProvider back-compat pattern.

Proposed cleanup

Add a TypeProvider-level API, such as BuildAttributesForBackCompatibility, that handles:

• restoring compatible attributes from the last contract,
• merging generated and custom-code attributes,
• filtering CodeGen-specific attributes that should not be preserved,
• deduplicating attributes consistently.

Then update CanonicalTypeProvider to use that shared API instead of owning this local merge/deduplication logic.

Notes

This was raised as a non-blocking review follow-up in PR #10976: #10976 (comment)

[github-actions]

🎯 Issue Triage

This is a refactoring/cleanup request to centralize type attribute back-compat merging logic into a shared TypeProvider-level API, rather than keeping it local to CanonicalTypeProvider.BuildAttributes().

Category: Feature Request
Area: emitter:client:csharp

🔍 Analysis

• Origin: Raised as a non-blocking follow-up from PR Use provider C# reference map with pre-write internalization #10976 review discussion.
• Current behavior: CanonicalTypeProvider.BuildAttributes() locally handles merging of generated, last-contract, and custom-code type attributes.
• Proposed improvement: Extract this logic into a TypeProvider.BuildAttributesForBackCompatibility() API to centralize responsibility and follow existing back-compat patterns.
• Scope: Internal refactor — no user-facing behavior change expected. Improves code consistency and maintainability in the C# emitter.
• Affected components: CanonicalTypeProvider, TypeProvider in http-client-csharp.

📝 Suggested next steps

• Implement BuildAttributesForBackCompatibility on TypeProvider handling restore, merge, filter, and deduplication
• Update CanonicalTypeProvider.BuildAttributes() to delegate to the new shared API
• Verify existing tests pass; add tests for the new API if needed

Generated by Agentic Triage