Copy plugins when attaching a model.

Before this CL, the same mjCPlugin objects were referenced by both models.

Fixes #2100.

PiperOrigin-RevId: 680579479
Change-Id: Ida3459f78f9fa4a942cea31b3d5d3e8da2832feb

src/user/user_model.h

@@ -386,7 +386,6 @@ class mjCModel : public mjCModel_, private mjSpec {
 
   mjListKeyMap ids;   // map from object names to ids
   mjCError errInfo;   // last error info
-  bool plugin_owner;  // this class allocated the plugins
   std::vector<mjKeyInfo> key_pending_;  // attached keyframes
 };
 #endif  // MUJOCO_SRC_USER_USER_MODEL_H_