Dedup purchase requests.

Since the theater commander runs once per campaign action, missions that
do not have aircraft available may be checked more than once a turn.
Without deduping requests this can lead to cases where the AI buys
dozens of tankers on turn 0.

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/1470

game/commander/tasks/compound/protectairspace.py

@@ -9,4 +9,4 @@ class ProtectAirSpace(CompoundTask[TheaterState]):
     def each_valid_method(self, state: TheaterState) -> Iterator[Method[TheaterState]]:
         for cp, needed in state.barcaps_needed.items():
             if needed > 0:
-                yield [PlanBarcap(cp)]
+                yield [PlanBarcap(cp, needed)]