Core/AI: Finally move the "is creature engaged" flag to be a property of the creature AI, where it honestly always belonged. Fixes #17981 and #23602 for real this time.

(cherry picked from commit 0e7c66cb4c)

src/server/scripts/Maelstrom/Stonecore/stonecore.cpp

@@ -173,7 +173,7 @@ class npc_sc_millhouse_manastorm : public CreatureScript
 
                 me->RemoveAllAuras();
                 me->CombatStop(true);
-                me->GetThreatManager().NotifyDisengaged();
+                EngagementOver();
 
                 switch (pointId)
                 {