## Why
Blocking PTY reads and output-channel sends can keep Tokio runtime shutdown
waiting when a detached child retains the terminal or output backpressure fills
the channel.
## What changed
- Drive Unix PTY reads and writes through nonblocking `AsyncFd` readiness so
their tasks can be cancelled during shutdown.
- Keep draining child output after its receiver closes, and preserve queued
input plus EOF delivery when portable PTY stdin closes.
- Return a descriptive error when PTY spawning uses a Tokio runtime without an
I/O driver.
## Testing
Add Unix coverage for detached children, full and dropped output channels,
large input with EOF, inherited file descriptors, and runtimes without I/O.
GitOrigin-RevId: f7f1f58abebf8bf1d39285cd61b8d69946d54155
bf3eb2ec91