delete_snapshot_tree removed the parent and every child snapshot individually.
On a real pool `zfs snapshot -r` creates one snapshot per descendant dataset --
252 on Tap -- so cleanup was 252 sequential middleware calls.
Slow, but the real problem is that it is not atomic: a job killed part-way
through the sweep leaves behind exactly the orphaned snapshots this function
exists to prevent.
zfs.snapshot.delete accepts {"recursive": True}, which destroys the parent and
all children in one call. Use that as the fast path and keep the name-by-name
sweep as the fallback -- it is still needed when the parent is already gone
(stock's finally can win the race once our mounts are released), which makes a
recursive delete fail while the children survive.
The test fake now emulates real `zfs destroy -r` semantics, so a test cannot pass
while the shipped code deletes only the parent.
76 tests, ruff and shellcheck clean.
flan
8421a34d8d