Fix pet stopping approach when attacked by another creature

Fix pet placed on stay after "attack" not attacking new attackers

Closes #8735

src/server/game/Entities/Unit/Unit.cpp

@@ -12136,7 +12136,10 @@ void Unit::CombatStart(Unit* target, bool initialAggro)
         if (!target->isInCombat() && target->GetTypeId() != TYPEID_PLAYER
             && !target->ToCreature()->HasReactState(REACT_PASSIVE) && target->ToCreature()->IsAIEnabled)
         {
-            target->ToCreature()->AI()->AttackStart(this);
+            if (target->isPet())
+                target->ToCreature()->AI()->AttackedBy(this); // PetAI has special handler before AttackStart()
+            else
+                target->ToCreature()->AI()->AttackStart(this);
         }
 
         SetInCombatWith(target);