Site Health: Deduplicate VCS detection via WP_Automatic_Updater (#47428)

[pranav-inamdar]

Summary

Removes duplicated VCS checkout detection between Site Health and the automatic updater by centralizing the filesystem walk and .svn / .git / .hg / .bzr checks.

Changes

• Adds WP_Automatic_Updater::get_vcs_checkout_details() to return whether a VCS metadata directory was found, which ancestor path matched, and which marker (e.g. .git) was used—before the automatic_updates_is_vcs_checkout filter is applied.
• Refactors WP_Automatic_Updater::is_vcs_checkout() to call get_vcs_checkout_details() and then apply the filter to the raw boolean, preserving existing behavior.
• Updates WP_Site_Health_Auto_Updates::test_vcs_abspath() to use get_vcs_checkout_details( ABSPATH ) for the user-facing messages and applies automatic_updates_is_vcs_checkout once with the raw discovery result (equivalent to the previous logic when a checkout was detected).
• Extends PHPUnit coverage in tests/phpunit/tests/admin/wpAutomaticUpdater.php for the new helper and consistency between raw discovery and the filtered is_vcs_checkout() result.

Trac ticket: https://core.trac.wordpress.org/ticket/47428

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