Fix AI landing behavior.

The landing waypoints need the airdrome_id field set to actually
associate with the airfield. Without this ferry flights will take off
and immediately land at their departure airfield.

game/theater/controlpoint.py

@@ -687,6 +687,10 @@ class ControlPoint(MissionTarget, ABC):
     ) -> RunwayData:
         ...
 
+    @property
+    def airdrome_id_for_landing(self) -> Optional[int]:
+        return None
+
     @property
     def parking_slots(self) -> Iterator[ParkingSlot]:
         yield from []
@@ -904,6 +908,10 @@ class Airfield(ControlPoint):
         assigner = RunwayAssigner(conditions)
         return assigner.get_preferred_runway(self.airport)
 
+    @property
+    def airdrome_id_for_landing(self) -> Optional[int]:
+        return self.airport.id
+
     @property
     def parking_slots(self) -> Iterator[ParkingSlot]:
         yield from self.airport.parking_slots