Fix: Delete entry-level file syncs when deleting a thumb asset

[devin-ai-integration]

Summary

When a thumb asset is deleted via ThumbAssetService::deleteAction, the entry-level file sync (created by setAsDefaultThumbAsset via kFileSyncUtils::createSyncFileLinkForKey) remains active. This happens because:

1. deleteAction soft-deletes the thumb asset (status → DELETED)
2. The kObjectDeletedEvent triggers syncableDeleted which deletes the asset-level file syncs
3. In deleteSyncFileForKey, when the source file sync has linkCount > 0, convertLinksToFiles() converts the entry's LINK file sync into an independent FILE — keeping it with status READY
4. thumbnailAction::execute() still serves the thumbnail because it resolves the file from the entry's file sync (now an independent file)

Fix: Added deleteEntryFileSyncsLinkedToAsset() which is called before the thumb asset is soft-deleted. It:

• Finds the thumb asset's file syncs
• Queries for entry-level file syncs (FileSyncObjectType::ENTRY, kEntryFileSyncSubType::THUMB) that have linked_id pointing to the thumb asset's file syncs
• Deletes those entry-level file syncs first, so convertLinksToFiles won't preserve stale thumbnail data

Review & Testing Checklist for Human

• Verify that deleting a non-default thumb asset makes the thumbnail inaccessible via the entry-level URL (/thumbnail/entry_id/{id}/version/{version})
• Verify that deleting a thumb asset does NOT affect the current default thumb asset's accessibility
• Verify that after deleting a thumb asset, the entry's current default thumbnail (different version) still works
• Check that when a thumb asset has file syncs across multiple DCs, the entry-level file syncs for all DCs are cleaned up
• Test the scenario where the thumb asset was previously the default but is no longer (TAG_DEFAULT_THUMB was removed before deletion)

Notes

• The fix only affects non-default thumb assets (the deleteAction already throws THUMB_ASSET_IS_DEFAULT if the asset has TAG_DEFAULT_THUMB)
• The deleteEntryFileSyncsLinkedToAsset method queries by LINKED_ID to find entry file syncs that are links to this specific thumb asset, so it won't affect unrelated entry file syncs
• The method is called before save() to ensure the entry file syncs are deleted before the kObjectDeletedEvent cascade runs convertLinksToFiles

Link to Devin session: https://app.devin.ai/sessions/fdc0eae331b54163b0359394b033c83f
Requested by: @bw-kaltura

[devin-ai-integration]

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