Support experimental model picker categories

[andysalerno]

Summary

• recognize the runtime's experimental model picker category in generated Rust types
• keep the provisional supplemental-model RPC untyped through Client::call
• verify raw response decoding and inspectable method-not-found fallback

Compatibility

This adds no first-class supplemental RPC API. Older runtimes continue returning JSON-RPC -32601, and unknown future categories still decode as Unknown.

Testing

• cargo test --manifest-path rust/Cargo.toml --features test-support --test session_test raw_client_call_supports_supplemental_model_compatibility

[Copilot]

Pull request overview

Adds Rust compatibility for experimental supplemental model categories while retaining raw RPC access and fallback handling.

Changes:

• Generates an Experimental model-picker category.
• Tests supplemental model decoding, unknown categories, and -32601 errors.

Show a summary per file

File	Description
scripts/codegen/rust.ts	Extends the Rust schema before generation.
rust/src/generated/api_types.rs	Adds the generated enum variant.
rust/tests/session_test.rs	Verifies raw RPC compatibility behavior.

Review details

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• Files reviewed: 2/3 changed files
• Comments generated: 0
• Review effort level: Balanced

[andysalerno]

Companion implementation: github/copilot-agent-runtime#16936 and github/github-app#13436. This PR intentionally adds category compatibility only; the supplemental RPC remains private and is invoked through generic Client::call.

[SteveSandersonMS]

Thanks for tackling the Rust side of this and for the clear test coverage on the raw Client::call compatibility path!

Since ModelPickerCategory is a shared protocol enum, we'd like to keep all generated SDKs aligned rather than patching this into Rust codegen alone. Could you extend the same schema change (and regeneration) to Node, C#, Python, Go, and Java as well, with per-language decoding/behavior tests for the new experimental value and the unknown-category fallback? E2E coverage isn't necessary here unless existing conventions in a given language already call for it — unit/integration-level tests matching what you've done for Rust should be sufficient.

Marking this as draft and adding waiting on response in the meantime — please mark it ready for review once the other languages are updated. Thanks!