mavlink_mission: robustify against dropped ack

This commit handles incoming MISSION_REQUEST_LIST messages such that
they are no longer ignored if the previously requested mission has
sent all mission items but the ack has never been received.

This was the cause for frequent "IGN REQUEST LIST: Busy" warnings

src/modules/mavlink/mavlink_mission.cpp

@@ -691,6 +691,14 @@ MavlinkMissionManager::handle_mission_request_list(const mavlink_message_t *msg)
 	mavlink_msg_mission_request_list_decode(msg, &wprl);
 
 	if (CHECK_SYSID_COMPID_MISSION(wprl)) {
+		const bool maybe_completed = (_transfer_seq == current_item_count());
+
+		// If all mission items have been sent and a new mission request list comes in, we can proceed even if  MISSION_ACK was
+		// never received. This could happen on a quick reconnect that doesn't trigger MAVLINK_MISSION_PROTOCOL_TIMEOUT_DEFAULT
+		if (maybe_completed) {
+			switch_to_idle_state();
+		}
+
 		if (_state == MAVLINK_WPM_STATE_IDLE || (_state == MAVLINK_WPM_STATE_SENDLIST
 				&& (uint8_t)_mission_type == wprl.mission_type)) {
 			_time_last_recv = hrt_absolute_time();