Prevent deletion from an attached mjSpec.

This change disallows calling `mjs_delete` on an mjSpec that has been attached to another mjSpec via `mjs_attach`. Attempting to delete an element from an attached spec will now result in an error. The Python bindings for `mjSpec.delete` have been updated to raise a ValueError when this occurs.

PiperOrigin-RevId: 897266695
Change-Id: Ic0670125a3028191ec50eca890f02b5910ec8b03

src/user/user_api.cc

@@ -474,8 +474,12 @@ int mj_copyBack(mjSpec* s, const mjModel* m) {
 // remove body from mjSpec, return 0 on success
 int mjs_delete(mjSpec* s, mjsElement* element) {
   mjCModel* model = static_cast<mjCModel*>(s->element);
+  if (model->IsAttached()) {
+    model->SetError(mjCError(nullptr, "Cannot delete element from an attached mjSpec."));
+    return -1;
+  }
   if (!element) {
-    model->SetError(mjCError(0, "Element is null."));
+    model->SetError(mjCError(nullptr, "Element is null."));
     return -1;
   }
   try {

src/user/user_model.h

@@ -327,6 +327,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   // set attached flag
   void SetAttached(bool deepcopy) { attached_ |= !deepcopy; }
 
+  // check if model is attached
+  bool IsAttached() const { return attached_; }
+
   // check for repeated names in list
   void CheckRepeat(mjtObj type);