Group inbound webhooks often report only the sender in participants,
not the full roster. Sweeping on any non-empty list incorrectly marked
approved members as left, silently dropping them from channel fan-out.
Gate the sweep on participantsAuthoritative so adapters opt in only when
the provider supplies a complete roster.
Co-authored-by: cynarAI <cynarAI@users.noreply.github.com>
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