[Eric Erhardt (eerhardt)]

Summary

• Prepend StreamJsonRpc's source-generated STJ metadata directly in MassageUserDataSerializerOptions when the formatter is constructed or assigned options. Cancellation parameters work even when the application's context omits RequestId.
• Preserve default reflection serialization when no resolver was supplied, without adding a reflection resolver to explicitly configured source-only options.
• Use ordinary order-based configuration: built-in metadata precedes existing resolvers, callers can prepend a custom resolver afterward, and subsequently replacing TypeInfoResolver removes the built-in registration. No Lazy, custom locking, or deferred options preparation.
• Update NativeAOT guidance and samples to assign fully configured options with JsonSerializerOptions = new() { TypeInfoResolver = MyContext.Default }.
• Cover eager registration, resolver precedence and replacement, caller-option isolation, missing application metadata, cancellation delivery, and NativeAOT execution.

Related to #1407. This PR contains only automatic metadata registration. Making RequestIdSTJsonConverter public and testing direct consumer source generation is isolated in #1557. The converter remains internal here; both changes target main independently.

Validation

• Release test-project build: zero warnings/errors.
• 789 targeted formatter/RPC tests passed across net8.0, net9.0, and net472, excluding known unstable tests (TestCategory=FailsInCloudTest).
• Updated samples build and shipping-library package/API validation passed with zero warnings/errors.
• Windows x64 NativeAOT publish and smoke-program execution passed with the application context omitting RequestId.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is focused, documented, and comprehensively covers the relevant resolver, cancellation, and NativeAOT scenarios.

Review effort: Balanced
Findings: None

What changed in this PR

Adds deferred built-in System.Text.Json metadata fallback, enabling cancellation serialization with application-defined source-generation contexts while preserving resolver precedence.

Changes:

• Lazily appends built-in metadata and freezes serializer options on first use.
• Adds formatter, cancellation, and NativeAOT regression coverage.
• Documents resolver-chain and reflection-fallback behavior.

File	Description
src/​StreamJsonRpc/​SystemTextJsonFormatter.cs	Implements deferred built-in metadata fallback.
test/​StreamJsonRpc.Tests/​SystemTextJsonFormatterTests.cs	Tests resolver precedence and options behavior.
test/​StreamJsonRpc.Tests/​JsonRpcSystemTextJsonHeadersTests.cs	Tests remote cancellation with generated metadata.
test/​NativeAOTCompatibility.Test/​SystemTextJson.cs	Adds NativeAOT RequestId coverage.
docfx/​docs/​nativeAOT.md	Documents metadata resolver requirements.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is consistent with the documented resolver semantics and has comprehensive targeted coverage.

Review effort: Balanced
Findings: None

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is focused, correctly documented, and comprehensively tested across resolver and NativeAOT scenarios.

Review effort: Balanced
Findings: None

[Andrew Arnott (AArnott)]

/azp run

[azure-pipelines]

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Successfully started running 1 pipeline(s).

[Andrew Arnott (AArnott)]

Recalling my prior approval because I now see that my concern about a breaking change is proven by your having to change the tests and sample so they keep working.
I'm still evaluating.

[Andrew Arnott (AArnott)]

Approved. I was mistaken about the breaking change, as I thought the change was in the library when it was only in a sample.
Can you resolve the merge conflict though?