Fix antag objectives always overshooting MaxDifficulty (and kill tries20) (#29830)

* The death of try20

* Add integration test for traitor gamerule

* Fix max difficulty being overshot

* Check at least one objective is assigned

* EntProtoId

Content.Shared/Random/Helpers/SharedRandomExtensions.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Content.Shared.Dataset;
 using Content.Shared.FixedPoint;
@@ -87,6 +88,26 @@ namespace Content.Shared.Random.Helpers
             throw new InvalidOperationException("Invalid weighted pick");
         }
 
+        public static T PickAndTake<T>(this IRobustRandom random, Dictionary<T, float> weights)
+            where T : notnull
+        {
+            var pick = Pick(random, weights);
+            weights.Remove(pick);
+            return pick;
+        }
+
+        public static bool TryPickAndTake<T>(this IRobustRandom random, Dictionary<T, float> weights, [NotNullWhen(true)] out T? pick)
+            where T : notnull
+        {
+            if (weights.Count == 0)
+            {
+                pick = default;
+                return false;
+            }
+            pick = PickAndTake(random, weights);
+            return true;
+        }
+
         public static (string reagent, FixedPoint2 quantity) Pick(this WeightedRandomFillSolutionPrototype prototype, IRobustRandom? random = null)
         {
             var randomFill = prototype.PickRandomFill(random);