Add XEP-0410 MUC self-ping to auto-rejoin after silent eviction

Add XEP-0410 MUC self-ping to auto-rejoin after silent eviction

A c2s reconnect re-fires session_start and rejoins the rooms, but a
silent MUC eviction (e.g. an s2s flap when the account server restarts)
never touches the c2s stream: the bot stays online and JID-pingable yet
quietly drops out of the room with no way to notice.

Periodically ping our own occupant JID and rejoin on not-acceptable /
item-not-found; treat every other error (and timeouts) as inconclusive
so a flaky s2s link can't cause a rejoin storm.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>