## What changed
- Add `CloudClient` and integrate it with `AppServerClient` and `AppServerRequestHandle` for requests, notifications, server-request replies, events, and shutdown.
- Keep retained request and reply handles bound to their cloud backend, with event delivery independent of request completion and a timeout for shutdown.
- Return no platform or Codex home metadata for cloud clients, and reject TUI filesystem operations for this backend.
## Testing
Add tests for backend ownership of retained handles, independent event delivery, errors after the command receiver closes, and shutdown timeouts when the worker does not respond.
GitOrigin-RevId: 4cf308dc0ea6fa6e7b14dd4542d93e8030b0162d
