* fix: ensure num_participants is accurate in webhook events (#4265)
Three fixes for stale/incorrect num_participants in webhook payloads:
1. Move participant map insertion before MarkDirty in join path so
updateProto() counts the new participant.
2. Use fresh room.ToProto() for participant_joined webhook instead of
a stale snapshot captured at session start.
3. Remove direct NumParticipants-- in leave path (inconsistent with
updateProto's IsDependent check), force immediate proto update,
and wait for completion before triggering onClose callbacks.
* fix: use ToProtoConsistent for webhook events instead of forcing immediate updates
cdb0769c38