Remove plugins that are not referenced after detaching a body.

Fixes #2497.

Also, use `Release()` instead of `delete` for removing elements when detaching a body in order to preserve correct reference count.

PiperOrigin-RevId: 739133888
Change-Id: I7ccfad84446fb15259bad30c4ba1e6f7fa601518

src/user/user_model.h

@@ -350,6 +350,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   void CopyPlugins(mjModel*);           // copy plugin data
   int CountNJmom(const mjModel* m);     // compute number of non-zeros in actuator_moment matrix
 
+  // remove plugins that are not referenced by any object
+  void RemovePlugins();
+
   // objects created here
   std::vector<mjCFlex*>     flexes_;      // list of flexes
   std::vector<mjCMesh*>     meshes_;      // list of meshes
@@ -430,6 +433,11 @@ class mjCModel : public mjCModel_, private mjSpec {
   // return true if body has valid mass and inertia
   bool CheckBodyMassInertia(mjCBody* body);
 
+  // Mark plugin instances mentioned in the list
+  template <class T>
+  void MarkPluginInstance(std::unordered_map<std::string, bool>& instances,
+                          const std::vector<T*>& list);
+
 
   mjListKeyMap ids;   // map from object names to ids
   mjCError errInfo;   // last error info