Cancel a stalled standard-input writer when the child process exits

[iCharlesHu]

When run() feeds standard input from a source that can stall and the child process exits before consuming that input, run() could hang indefinitely. The input-writing task was parked awaiting its source rather than blocked inside a pipe write, so the existing cancelAsyncIO path (which only unblocks in-flight pipe I/O) could not release it, and the I/O task group never finished draining. Spawn a waitForProcessTermination waiter alongside the input-writing task that cancels the I/O group once the child exits.

Also add edge-case coverage for process I/O and AsyncIO, ported from swift-async-process and swift-nio and replace FileDescriptor.close with _safelyClosed.