[http-client-csharp] Preserve error response names and isolate generic keep registrations

[Copilot]

Automatic Response → Result normalization renamed error contracts and exposed a keep-key collision: preserving the internal ErrorResult<T> helper also retained unrelated non-generic ErrorResult models, introducing unintended public API.

• Error-model naming: Exempt models marked with error usage from suffix normalization. Preserve ordinary response normalization and explicit naming customizations.
• Keep-set identity: Reuse the reference map’s arity-aware identity when materializing provider registrations, keeping ErrorResult and ErrorResult1` distinct while preserving lazy registration.
• Regression coverage: Cover referenced and pruned error models, generic/non-generic isolation for root and non-root registrations, helper visibility, and model-factory/context cleanup.

• Fixes [http-client-csharp] Preserve error response model names and fix the ErrorResult helper keep-key collision #12048

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commit: 7beb863

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation addresses both linked root causes with comprehensive regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes error-model renaming and generic keep-key collisions in the C# client generator.

Changes:

• Preserves Response suffixes for error models.
• Uses generic-arity-aware keep identities.
• Adds naming, pruning, visibility, factory, context, and arity regression tests.

File	Description
ModelProvider.cs	Exempts error models from suffix normalization.
CodeModelGenerator.cs	Materializes arity-aware keep keys.
ProviderReferenceMapAnalyzer.BodyReferences.cs	Shares provider identity generation.
ModelProviderTests.cs	Tests error naming and customization.
ProviderReferenceMapAnalyzerTests.cs	Tests generic arity isolation.
ClientBodyDependencyReferenceMapTests.cs	Tests end-to-end pruning and helper isolation.

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