Holding expiry or peer teardown can reclaim an Update Bearer transaction
while it is still linked to its MME bearer, leaving a stale reference to
a recycled pool slot.
Add an optional delete_cb invoked before transaction resources are
released. The MME callback uses the bearer ID stored in peer_data and
unlinks the node only if the bearer exists and the node is still present.
This handles prior unlinking and bearer removal without double removal.
Add a regression through the Update Bearer handler, holding timer expiry,
and paging continuation. Verify that no stale transaction remains and no
NAS modification or peer timer is started after expiry.