Relaunch the debug shell if user exits it.

[benhillis]

While looking at #13330 I realized that it's not ideal that the debug shell will become unusable if the user runs "exit" in it. Resolve that issue by adding a relaunch loop.

This does not actually fix the reported issue, but I think is a useful change to take in isolation as it improves debugging capability.

[Copilot]

Pull Request Overview

This PR improves the WSL debug shell by adding automatic relaunch functionality. When a user exits the debug shell (e.g., by typing "exit"), the shell will automatically restart instead of becoming permanently unusable.

• Wraps the debug shell process in a relaunch loop that monitors and restarts the shell when it exits
• Uses proper process management with waitpid to handle child process termination
• Maintains the same agetty configuration for the actual shell process