Fix: delay plan9 registration after OOBE

[chemwolf6922]

Summary of the Pull Request

Currently, after a fresh distro installation, operations in the linux plan9 share will use the uid 0. Because the uid was cached before the OOBE is complete. And this only recovers after a distro termination.
This PR delays the plan9 registration after the OOBE completes with the correct default uid.

PR Checklist

• Closes: Windows apps writing to \\wsl.localhost create files owned by root (UID 0) instead of WSL default user #40941
• 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

Updated the test UnitTests::UnitTests::ModernOOBE to verify this change.

[Copilot]

Pull request overview

This PR fixes a WSL2 Plan9/\\wsl.localhost ownership regression on fresh installs by deferring Plan9 redirector connection-target registration until after OOBE completes, so Windows-initiated file operations use the correct default UID instead of a pre-OOBE cached value (often UID 0).

Changes:

• Register the Plan9 redirector connection target after successful OOBE completion (once DefaultUid is known).
• Skip Plan9 connection-target registration while OOBE is still pending for the distro.
• Extend ModernOOBE unit test coverage to verify \\wsl.localhost access is blocked pre-OOBE and that files created post-OOBE have the expected UID.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/windows/service/exe/WslCoreInstance.cpp	Delays Plan9 connection-target registration until after successful OOBE, ensuring correct UID is used.
test/windows/UnitTests.cpp	Updates ModernOOBE test to validate pre-/post-OOBE \\wsl.localhost behavior and resulting file ownership.