## Why
Remote process exit can arrive before the final output, and inherited pipes can keep the output stream open after exit. Removing the session at exit can lose output that should remain available to `write_stdin` polls.
## What changed
- Retain remote sessions with open or unread output after exit, including early exits and cancelled initial collection.
- Collect pending output when `write_stdin` races with exit, skipping stdin writes once remote exit is known.
- After initial collection finishes and the process exits, allow a five-second grace period before a final background read. Respect active polling deadlines and keep later polls available while that read recovers retained output.
- Let explicit termination stop output collection even when the process has already exited, and install the exit watcher before cancellable cleanup.
## Testing
Extend remote process event tests to cover output arriving after the initial yield, polling during a delayed final read, active polls exceeding the background grace period, resumable cancellation, and termination before the initial yield.
GitOrigin-RevId: b1671fad8b61a2cdecf7cf9fdf92e57c028c7af8
jif
b043762f9e