chore: clean up TypeProvider attributes

[jorgerangel-msft]

fixes: #11232

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commit: caddb16

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[Copilot]

Pull request overview

This PR refactors how TypeProvider handles attributes to better separate (1) attributes that can be cached/analyzed during reference-map analysis from (2) attributes that must be rebuilt at write time based on finalized generation decisions (notably the MRW context’s buildable attributes). It also simplifies the reference-map helper code and adjusts MRW tests to ensure back-compat attribute materialization happens before assertions.

Changes:

• Remove an unused name-matching helper in ProviderReferenceMapAnalyzer.Helpers.
• Rework TypeProvider attribute write-time behavior by introducing a rebuild path (GetAttributesForWrite + RebuildAttributes) and a new back-compat hook (BuildAttributesForBackCompatibility).
• Update ModelReaderWriterContextDefinition to apply last-contract buildable attribute restoration via BuildAttributesForBackCompatibility, and update tests to call ProcessTypeForBackCompatibility() before inspecting buildable attributes.

Reviewed changes

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File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/ReferenceMap/ProviderReferenceMapAnalyzer.Helpers.cs	Removes an unused private helper.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/TypeProvider.cs	Introduces write-time attribute rebuild support and a back-compat hook for attributes.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/ModelReaderWriterContextDefinition.cs	Moves last-contract buildable attribute restoration into the new attribute back-compat hook and caches customized buildable types.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/Definitions/ModelReaderWriterContextDefinitionTests.cs	Ensures tests materialize back-compat state before asserting on buildable attributes.