fix(files_versions): avoid version snapshot races during cross-storage renames

[joshtrichards]

• Resolves: # (possibly some existing bug reports; see user-visible symptoms list below)

Summary

Cross-storage moves already have dedicated version handling via VersionStorageMoveListener, which handles moving existing versions across backends.

At the same time, FileEventsListener::write_hook() can still call Storage::store() while that move is in progress. For moves between backends, that is redundant and can race with the move flow by snapshotting the source path while it is already being moved.

Changes:

• track source paths for active cross-backend moves in FileEventsListener
• skip Storage::store() in write_hook() for writes under those paths
• clear the tracked path again when the move completes

I found this while investigating intermittent failures in apps/files_trashbin/tests/StorageTest.php::testKeepFileAndVersionsWhenMovingFileBetweenStorages. The existing test already covers the affected scenario; the problem is the race in the implementation, not missing scenario coverage.

Possible user-visible symptoms:

• intermittent failure when moving a file between storages
• generic move/file operation error caused by version creation racing with the move

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

Example failure from an actual CI run (https://github.com/nextcloud/server/actions/runs/27333716311/job/80752712482?pr=61185):

There was 1 error:

1) OCA\Files_Trashbin\Tests\StorageTest::testKeepFileAndVersionsWhenMovingFileBetweenStorages
RuntimeException: Failed to copy to files_versions/test.txt.v1781170610 from cache with source data {...} 

/home/runner/actions-runner/_work/server/server/lib/private/Files/Cache/Cache.php:1262
...
/home/runner/actions-runner/_work/server/server/apps/files_versions/lib/Versions/LegacyVersionsBackend.php:163
/home/runner/actions-runner/_work/server/server/apps/files_versions/lib/Versions/VersionManager.php:96
/home/runner/actions-runner/_work/server/server/apps/files_versions/lib/Storage.php:217
/home/runner/actions-runner/_work/server/server/apps/files_versions/lib/Listener/FileEventsListener.php:210
/home/runner/actions-runner/_work/server/server/apps/files_versions/lib/Listener/FileEventsListener.php:83
...
/home/runner/actions-runner/_work/server/server/apps/files_trashbin/tests/StorageTest.php:507

[joshtrichards]

/backport to stable34

[joshtrichards]

/backport to stable33