Core/AI: Implemented OnHealthDepleted hook

Port From (https://github.com/TrinityCore/TrinityCore/commit/ea134c0eae83b4cdbb5f0e941ed867a144bb01f6)

Source/Game/AI/CoreAI/CreatureAI.cs

@@ -448,6 +448,9 @@ namespace Game.AI
         // Called for reaction when initially engaged - this will always happen _after_ JustEnteredCombat
         public virtual void JustEngagedWith(Unit who) { }
 
+        // Called when the creature reaches 0 health (or 1 if unkillable).
+        public virtual void OnHealthDepleted(Unit attacker, bool isKill) { }
+
         // Called when the creature is killed
         public virtual void JustDied(Unit killer) { }
 

Source/Game/Entities/Unit/Unit.Combat.cs

@@ -942,7 +942,10 @@ namespace Game.Entities
                 // Call creature just died function
                 CreatureAI ai = creature.GetAI();
                 if (ai != null)
+                {
+                    ai.OnHealthDepleted(attacker, true);
                     ai.JustDied(attacker);
+                }
 
                 TempSummon summon = creature.ToTempSummon();
                 if (summon != null)

Source/Game/Entities/Unit/Unit.cs

@@ -2672,6 +2672,10 @@ namespace Game.Entities
             else if (victim.IsCreature() && damageTaken >= health && victim.ToCreature().HasFlag(CreatureStaticFlags.Unkillable))
             {
                 damageTaken = health - 1;
+
+                // If we had damage (aka health was not 1 already) trigger OnHealthDepleted
+                if (damageTaken > 0)
+                    victim.ToCreature().GetAI()?.OnHealthDepleted(attacker, false);
             }
             else if (victim.IsVehicle() && damageTaken >= (health - 1) && victim.GetCharmer() != null && victim.GetCharmer().IsTypeId(TypeId.Player))
             {