Fix helicopters at airfields breaking generation.

Helipads at airfields don't work well right now because they are not
counted as additional parking, but we should still allow them to take
off from the airfield parking.

Follow up work would be to fix the parking problem and allow adding
helipads to airfields, or maybe to just "ground" start helicopters at
airfields so they take off from parking rather than the runway.

cherry-pick from 2499276b2a

gen/aircraft.py

@@ -76,6 +76,7 @@ from game.theater.controlpoint import (
     Airfield,
     ControlPoint,
     ControlPointType,
+    Fob,
     NavalControlPoint,
     OffMapSpawn,
 )
@@ -778,22 +779,20 @@ class AircraftConflictGenerator:
                     start_type=flight.start_type,
                     at=carrier_group,
                 )
-            else:
-                # If the flight is an helicopter flight, then prioritize dedicated helipads
-                if flight.unit_type.helicopter:
-                    return self._generate_at_cp_helipad(
-                        name=name,
-                        side=country,
-                        unit_type=flight.unit_type.dcs_unit_type,
-                        count=flight.count,
-                        start_type=flight.start_type,
-                        cp=cp,
-                    )
-
-                if not isinstance(cp, Airfield):
+            elif isinstance(cp, Fob):
+                if not flight.unit_type.helicopter:
                     raise RuntimeError(
-                        f"Attempted to spawn at airfield for non-airfield {cp}"
+                        f"Cannot spawn fixed-wing aircraft at {cp} because it is a FOB"
                     )
+                return self._generate_at_cp_helipad(
+                    name=name,
+                    side=country,
+                    unit_type=flight.unit_type.dcs_unit_type,
+                    count=flight.count,
+                    start_type=flight.start_type,
+                    cp=cp,
+                )
+            elif isinstance(cp, Airfield):
                 return self._generate_at_airport(
                     name=name,
                     side=country,
@@ -802,6 +801,10 @@ class AircraftConflictGenerator:
                     start_type=flight.start_type,
                     airport=cp.airport,
                 )
+            else:
+                raise NotImplementedError(
+                    f"Aircraft spawn behavior not implemented for {cp} ({cp.__class__})"
+                )
         except Exception as e:
             # Generated when there is no place on Runway or on Parking Slots
             logging.error(e)