Merge-time fix for #676 (review nits 4 and 5). sessionCreated forgot a stale
case default inside the try whose catch rolls the registration back; it now
does so after the try, once the session is committed, so the cleanup can
never drop a committed session from the live map. caseDefaultGroup (behind
GET /api/cases/:name/default-group) no longer deletes a stale id as a side
effect: it reports Ungrouped and leaves the value for the next creation to
forget. The two tests that expected the read to clear it now expect it to
write nothing; the creation test still pins the cleanup.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>