Rust: add race-safe canvas availability waiter

[ellismg]

Summary

• add Session::wait_for_canvas(WaitForCanvasOptions) for waiting on a known canvas declaration without sending a model turn
• subscribe before the initial session.canvas.list snapshot, then re-list on registry changes or subscription lag
• support provider-specific matching, total timeouts, and deterministic session-shutdown errors
• cover existing declarations, registration races, lag recovery, timeout/no-turn behavior, and shutdown during both event waiting and an in-flight list request

Motivation

github/github-app#11029 currently has to coordinate session.canvas.list and session.canvas.registry_changed itself while asynchronously loaded extensions register their canvases. This helper provides the smallest SDK-level primitive that removes the list/subscription race when the caller already knows the target canvas.

The API deliberately guarantees only target availability. It does not claim that extension initialization or the complete canvas registry has settled. A handshake-backed bounded initialization snapshot remains a separate copilot-agent-runtime protocol follow-up for github/github-app#7544 and github/github-app#10224.

Compatibility

This is an additive Rust API implemented entirely over existing protocol-v3 methods and events. It does not change session.create, add wire fields, block session creation on extension startup, or require a model turn. Older runtimes therefore need no protocol changes.

CHANGELOG.md is release-generated and has no unreleased section, so this PR relies on public rustdoc rather than editing it manually.

Validation

• cargo +nightly-2026-04-14 fmt --all -- --config-path .rustfmt.nightly.toml --check
• cargo clippy --all-targets --features test-support -- --no-deps -D warnings -D clippy::unwrap_used -D clippy::disallowed_macros -D clippy::await_holding_invalid_type
• cargo test --features test-support

[Copilot]

Pull request overview

Adds a Rust helper for race-safe waiting on canvas declarations without starting a model turn.

Changes:

• Adds Session::wait_for_canvas with provider matching and timeout handling.
• Adds dedicated timeout and session-closed errors.
• Adds coverage for registration races, lag, shutdown, and timeout behavior.

Show a summary per file

File	Description
rust/src/session.rs	Implements the canvas availability waiter.
rust/src/types.rs	Defines waiter configuration options.
rust/src/errors.rs	Adds canvas-wait error variants and messages.
rust/tests/session_test.rs	Tests matching, races, lag, timeout, and shutdown.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Balanced