Fix systemd-tmpfiles failure on systemd v261+ with systemd boot disabled

[Feng Wang (chemwolf6922)]

Summary of the Pull Request

In systemd v261, the systemd's tmpfiles.d/x11.conf was changed from D! to D. systemd/systemd@5474cad.
This means the systemd-tmpfiles command will try to execute it when called out of the boot sequence. In the linked issue's case, it's called by the post install script by dpkg. This will fail as the wsl override is not in place when systemd is disabled.

This PR enables the wsl override file generation for all distros with wslg enabled, regardless of if the distro boots with systemd.

PR Checklist

• Closes: WSLg read-only /tmp/.X11-unix breaks dpkg --configure on distros not booting systemd (systemd-tmpfiles exit 73) #41488
• 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

Update tests:
UnitTests::UnitTests::GuiApplications

[Copilot]

Pull request overview

This PR addresses systemd-tmpfiles failures on systemd v261+ in WSLg-enabled distros that do not boot with systemd by ensuring WSL always generates a /run/tmpfiles.d/x11.conf override when the read-only X11 socket mount is established. This prevents distro-provided tmpfiles.d/x11.conf rules from attempting to chmod/chown the read-only /tmp/.X11-unix mount when systemd-tmpfiles is invoked outside the boot sequence (e.g., during dpkg postinst).

Changes:

• Move generation of the /run/tmpfiles.d/x11.conf override from systemd unit generation to X11 mount initialization so it applies regardless of whether systemd is used as PID 1.
• Update Windows unit tests to validate the override file exists and that systemd-tmpfiles --create --boot x11.conf succeeds under WSLg.
• Remove the previous override generation logic from GenerateSystemdUnits().

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/linux/init/config.cpp	Generates /run/tmpfiles.d/x11.conf during X11 mount setup for non-system distros, ensuring the override exists even when systemd boot is disabled.
src/linux/init/init.cpp	Removes the prior tmpfiles override generation from systemd unit generation, avoiding reliance on the systemd path.
test/windows/UnitTests.cpp	Adds coverage asserting the override file exists when WSLg is enabled and systemd-tmpfiles succeeds; asserts absence when WSLg is disabled.

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