Add optional response contract to Rust SDK messages

[ellismg]

Summary

• Add MessageOptions.response_optional: Option<bool> and with_response_optional(bool) and forward responseOptional through high-level Session::send.
• Expose the same optional flag in generated SendRequest, SendMessageItem, and UserMessageData types. Explicit false is serialized; unset values remain omitted.
• Preserve existing response requirements by default. Runtime permits silent completion only for explicitly optional inputs; required messages in a combined turn still require output, and execution/provider errors are unaffected.

Runtime dependency

Requires the paired runtime optional-notification-response implementation: https://github.com/github/copilot-agent-runtime/pull/22365 (validated runtime commit beea6d004ef). SDK transports the flag; runtime enforces behavior. App opts in only for My Copilot routine idle notifications; error, plan-ready, and direct user requests remain required.

Generated source

Regenerated Rust with unchanged pinned CLI 1.0.87-0 schemas plus only the authoritative responseOptional property additions from the runtime producer. Preserves upstream structured-output fields. No CLI pin or TypeScript SDK changes.

Validation

• Failing-first missing-API tests confirmed before implementation.
• Five focused true/false/omitted/default/idle-completion tests pass.
• Full Rust session target: 151 passed; formatting and whitespace checks pass.
• App coordinator reports 20 focused Rust tests and six real-stack empty-turn E2Es passing with SDK 2e77fb6c and runtime beea6d004ef.