## Why
Aborting the child waiter during termination can leave an exited PTY child
unreaped and prevent the session from recording its exit status.
## What changed
- Detach the child waiter instead of aborting it when terminating or dropping a
`ProcessHandle`, while continuing to abort the I/O helper tasks.
- Cover explicit termination and handle drops for pipe processes, plus queued
PTY waiters and PTY process-group termination on Unix.
GitOrigin-RevId: 313787f4edec80b30cf5a78f0cc6733f1520d9a4