rust: forward cached models during session creation

[Chuxel]

Why

The runtime already accepts a cached model catalog during session.create, but the Rust SDK does not expose or forward it. This leaves hosts unable to avoid the duplicate model-catalog fetch through the supported SDK API.

Related issue: https://github.com/github/copilot-agent-runtime/issues/17913
Runtime support (merged): https://github.com/github/copilot-agent-runtime/pull/18113
Changes

• Expose CachedModel { id: String, supports_reasoning_effort: bool } and SessionConfig.cached_models: Option<Vec<CachedModel>>, plus the normal with_cached_models builder.
• Forward only id and required supportsReasoningEffort in camelCase cachedModels entries. None omits the field, an empty list is authoritative and stays [], and false is preserved.
• Keep this input exclusive to session.create; session.resume is unchanged. The SDK performs no new fetch, validation, or refresh. The runtime retains ownership of the asynchronous post-create refresh, and the consumer retains account/catalog policy.
• Add four focused config-to-wire tests covering omission, empty and populated catalogs, and resume exclusion.

Only rust/src/types.rs and rust/src/wire.rs change. This PR targets latest upstream main; no release-prep targeting requirement was found. No consumer vendoring, dependency pins, runtime edits, or feature flags are included. The App must receive this API through normal canonical sync after SDK merge.

Validation

The new tests first failed to compile because the public API was missing. After implementation:

• Nightly formatting check passed: cargo +nightly-2026-04-14 fmt --all -- --config-path .rustfmt.nightly.toml --check.
• cargo clippy --all-features --all-targets -- -D warnings passed.
• cargo test --all-features --lib types::tests passed all 70 tests, including the four new contract tests.
• cargo test --all-features --no-run compiled the Rust unit and integration test targets.

Cargo validation used COPILOT_SKIP_CLI_DOWNLOAD=1 because these checks exercise SDK serialization and compilation, not runtime behavior. Runtime-backed E2E tests were not run.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused changes are tested and have no unresolved blocking issues.

Review tier: Lite (auto)
Findings: None

Note

Copilot is running an experiment and ran this review at Lite.

What changed in this PR

Adds Rust SDK support for forwarding cached model metadata during session.create.

Changes:

• Adds CachedModel and SessionConfig.cached_models.
• Preserves omission, empty-list, and camelCase serialization semantics.
• Adds focused wire-format tests while leaving session.resume unchanged.

File	Description
rust/​src/​wire.rs	Adds cachedModels to session creation payloads.
rust/​src/​types.rs	Defines the cached-model API, builder, conversion, and tests.

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