[http-client-csharp] Preserve reader options when deserializing external models

[Copilot]

Generated deserializers force wire options when reading external models, dropping unknown nested JSON properties during persistence round-trips.

• Options propagation: Pass the enclosing ModelReaderWriterOptions to fallback ModelReaderWriter.Read calls instead of hardcoding WireOptions. JSON mode retains unknown properties; wire behavior remains unchanged.
• Regression coverage: Cover external models in scalar, list, and dictionary properties across JSON/wire read-write combinations, and update the generated-method baseline.

• Fixes [http-client-csharp] Preserve reader options when deserializing external models #11913

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commit: c66cfa8

[Copilot]

🟢 Approval recommended

The change directly addresses the reported option-propagation bug with targeted regression coverage for external models across relevant property shapes and formats.

Pull request overview

Fixes a C# http-client generator bug where deserialization of external model types forced WireOptions, causing unknown JSON properties to be dropped during JSON round-trips. The change forwards the caller’s ModelReaderWriterOptions into the ModelReaderWriter.Read(...) fallback path, and adds a regression test that executes generated code to validate behavior across scalar/list/dictionary external-model properties and JSON/wire combinations.

Changes:

• Forward mrwOptions (instead of WireOptions) to ModelReaderWriterSnippets.Read(...) in the MRW deserialization fallback.
• Add a compilation+execution regression test validating unknown-property preservation only for JSON read+write (J/J) with external model properties (scalar/list/dictionary).
• Update the existing generated-method baseline and add test-only context shims to support the new scenario.

File summaries

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs	Fixes the fallback ModelReaderWriter.Read call to propagate the caller’s reader options (mrwOptions).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/MrwSerializationTypeDefinitionTests.cs	Adds an execution-based regression test covering external models and updates an existing baseline assertion to compare the emitted method text.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/MrwSerializationTypeDefinitionTests/GetUtf8BytesIsUsedForMrwFallback.cs	Updates the expected generated deserializer text to reflect option propagation and the new assertion shape.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/MrwSerializationTypeDefinitionTests/ExternalModelPropertiesPreserveReaderOptions(Context).cs	Adds test-only context/extension shims used by the regression scenario’s compilation.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

The change is a targeted bug fix aligned with the linked issue and is covered by generator-output regression tests verifying the corrected options propagation in the emitted deserializers.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite