Merge branch 'fix/channel-getparticipant-left-member'

GetParticipant only converted a stale "left" row into USER_NOT_PARTICIPANT
when the participant being queried was the viewer themselves. Querying
about a different user who previously left/was kicked returned their stale
row as a successful result instead, which a client can't classify as a
proper participant type. Check the status regardless of who's asking.

internal/store/postgres/channel_member_list.go

@@ -184,7 +184,12 @@ func (s *ChannelStore) GetParticipant(ctx context.Context, viewerUserID, channel
 	if errors.Is(err, domain.ErrChannelPrivate) {
 		return domain.ChannelMember{}, domain.ErrUserNotParticipant
 	}
-	if err == nil && participantUserID == viewerUserID && member.Status == domain.ChannelMemberLeft {
+	// A stale row for a former member (left/kicked) is not a current
+	// participant for any caller, not just a self-check: channels.getParticipant
+	// on someone who has left must surface USER_NOT_PARTICIPANT like a missing
+	// row does, or callers get a "successful" ChannelParticipantLeft result for
+	// what protocol-wise is an absent participant.
+	if err == nil && member.Status != domain.ChannelMemberActive {
 		return domain.ChannelMember{}, domain.ErrUserNotParticipant
 	}
 	return member, err