_collect_delegate_child_ids() walks the _delegate_from marker chain to
gather delegate subagents for cascade deletion, but started its visited
set empty. When the chain loops back onto a parent — a delegation cycle,
or a parent that is also another parent's delegate child when several ids
are deleted together — that parent was collected as one of its own
descendants and then permanently deleted, along with all of its messages,
by _delete_delegate_children().
Seed the visited set with the parent ids so they can never be re-collected,
and exclude them from the returned child set. Callers (delete_session,
bulk delete) remove the parents separately, so this only prevents the
unintended parent deletion; legitimate child collection is unchanged.
Add regression tests (in-memory sqlite) covering single/multi-level
delegate chains, the parent_session_id+marker branch, untagged children
(orphan-don't-delete contract), and the cycle case that previously leaked
the parent into the deletion set.
Fixes#49148
