Fix followers getting sent to nullspace when target is polymorphed (#33878)

* Add an event for polymorph actions

* Subscribe FollowerSystem to the event

* Add check that unfollow target is actually current target.

* Make followers try to follow on state update; Dirty follower on follow.

Content.Server/Polymorph/Systems/PolymorphSystem.cs

@@ -267,6 +267,10 @@ public sealed partial class PolymorphSystem : EntitySystem
         if (PausedMap != null)
             _transform.SetParent(uid, targetTransformComp, PausedMap.Value);
 
+        // Raise an event to inform anything that wants to know about the entity swap
+        var ev = new PolymorphedEvent(uid, child, false);
+        RaiseLocalEvent(uid, ref ev);
+
         return child;
     }
 
@@ -339,6 +343,10 @@ public sealed partial class PolymorphSystem : EntitySystem
         // if an item polymorph was picked up, put it back down after reverting
         _transform.AttachToGridOrMap(parent, parentXform);
 
+        // Raise an event to inform anything that wants to know about the entity swap
+        var ev = new PolymorphedEvent(uid, parent, true);
+        RaiseLocalEvent(uid, ref ev);
+
         _popup.PopupEntity(Loc.GetString("polymorph-revert-popup-generic",
                 ("parent", Identity.Entity(uid, EntityManager)),
                 ("child", Identity.Entity(parent, EntityManager))),