When items are removed during a library scan, their user data is
detached to a placeholder. If a replacement item already exists
(e.g., a new version of the same episode was added before the old
file was deleted), the user data would be stranded in the placeholder
because the replacement item's initial ReattachUserDataAsync call
happened before the old item was deleted.
This fix checks for remaining valid children that share user data
keys with removed items and reattaches any detached user data to them.
Fixes#16149
David Federman
7bf08daeec