Fix mechanism events not being called properly, add test (#2279)

* Add mechanism events when added/removed to/from body/parts

* Change old usages

* Add TODO

* Remove BodyExtensions and IHasBody

* Remove unnecessary extensions and fix wrong event call in mechanism behavior component

* Complete test and fix event calls

Content.Server/GameObjects/Components/Body/MechanismComponent.cs

@@ -47,7 +47,7 @@ namespace Content.Server.GameObjects.Components.Body
             PerformerCache = null;
             BodyCache = null;
 
-            if (eventArgs.Target.TryGetBody(out var body))
+            if (eventArgs.Target.TryGetComponent(out IBody? body))
             {
                 SendBodyPartListToUser(eventArgs, body);
             }