Avoid distro zombie state when wsl init dies in systemd mode

[chemwolf6922]

Summary of the Pull Request

This PR adds a wsl-init-watcher process to monitor the wsl init when systemd mode is enabled.
The mini_init process monitors the process 1 of a distro to determine if the distro is alive. In non-systemd mode, that is the wsl init. In systemd mode, that will be the systemd init. And if the wsl init dies in systemd mode, the distro ends up in a zombie state. Where all wsl calls from Windows fail with "Catastrophic failure".
This PR works by adding a new process that monitors the wsl init in systemd mode. And requests a shutdown if the wsl init process died.

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Add test: UnitTests::UnitTests::SystemdKillInitTerminatesDistro

[Copilot]

Pull request overview

This PR aims to prevent WSL distributions from getting stuck in a “zombie” state when wsl-init dies while systemd mode is enabled, by introducing a lightweight watcher process that monitors wsl-init and triggers a PID-namespace teardown when it exits.

Changes:

• Adds a new wsl-init-watcher entrypoint in the Linux init multicall binary and forks it when BootInit (systemd mode) is used.
• Implements watcher logic using pidfd_open + poll() to detect wsl-init exit and then calls reboot(RB_POWER_OFF) to tear down the PID namespace.
• Adds a Windows unit test that kills the presumed wsl-init PID in systemd mode and verifies subsequent WSL commands succeed.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
test/windows/UnitTests.cpp	Adds a unit test to validate the distro recovers after wsl-init is killed in systemd mode.
src/shared/inc/lxinitshared.h	Introduces the LX_INIT_WSL_INIT_WATCHER name constant for the new entrypoint.
src/linux/init/init.cpp	Adds wsl-init-watcher dispatch, forks the watcher in systemd mode, and implements watcher behavior via pidfd monitoring and namespace teardown.