Prevent run_cvd abort and keep monitor socket alive on graceful VM sh…

[SuperStrongDinosaur]

Problem

When a Cuttlefish guest VM gracefully powers off, crosvm terminates cleanly, which in turn leads the ProcessMonitor subprocess to stop all monitored processes and exit with code 0.

Previously, run_cvd treated any readability on the monitor status pipe as an not normal failure. This caused run_cvd to immediately abort and abandon launcher_monitor.sock. As a result:
1. The VM entered an Unreachable state .
2. Subsequent cvd restart, cvd stop, or cvd powerwash commands failed with Connection refused.

Solution

1. Keep run_cvd Server Loop Alive on Exit:
   - Track process_monitor_active. When the monitor exit pipe is triggered in Select(), mark process_monitor_active = false so subsequent iterations only poll server_ .
   - Immediately invoke process_monitor.StopMonitoredProcesses() to wait for and clean up the terminated monitor child process, preventing lingering <defunct> zombie processes.
   - Keep the server loop running to service subsequent launcher actions (cvd restart, cvd stop, cvd powerwash, and cvd status).

2. Harden StopMonitoredProcesses():
   - Make StopMonitoredProcesses() safe to call multiple times and handle cases where the monitor has already exited or finished.
   - Eliminate mid-shutdown race conditions: if SendEmptyRequest encounters a closed/shut down socket because the monitor child is already in its exit path, do not abort early. Instead, proceed to wait for the process to exit using waitpid(last_monitor, &wstatus, 0) and log abnormal exits as warnings rather than failing the stop/restart request.

• Verified lifecycle workflows:
• Graceful guest VM shutdown via VePSM / adb reboot -p leaves launcher_monitor.sock active and eliminates zombie processes.
• cvd restart successfully connects, reaps state, and re-executes run_cvd.
• cvd stop cleanly terminates run_cvd with exit code 0.
• cvd status and cvd fleet successfully respond without connection errors.

Bug: b/534717429