[codex] Fix room input stop lifecycle

[jinfeng66]

Summary

• Stop external room-input participants from the session stop route before deleting the LiveKit room.
• Resolve legacy xunfei /start control URLs to /stop so existing env configuration can be reused.
• Treat room-input stop failures as best-effort while keeping LiveKit room deletion as the fallback cleanup.
• Update session stop tests to cover the new cleanup order and nonfatal room-input failures.

Why

The xunfei input source owns external audio, video, face listener, and socket resources. Relying only on Agent shutdown or LiveKit room deletion could leave those resources closing too late, which made repeated sessions vulnerable to stale input state or missing audio.

Validation

• pnpm test -- --test-name-pattern "session stop|maps livekit|parses the latest"
• pnpm exec tsc --noEmit --pretty false
• pnpm exec prettier --check app/api/session/stop/route.ts tests/session-stop.test.mjs
• git diff --check

[jinfeng66]

Closing this draft because the change was pushed to the existing observability PR #12 instead.