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.