Add nested keyframes attachment test.

This is a no-op change for keyframes but it shows an example of behavior.

Also, fixed two bugs:
- Missing namespace when attaching frames.
- Incorrect body id and subtreedof when attaching a compiled spec. Fixes #2063.

PiperOrigin-RevId: 680289467
Change-Id: I2ddaa7ad2226fa466ec3bbf74a7553d497bb4bb9

src/user/user_api.cc

@@ -119,6 +119,10 @@ mjModel* mj_compile(mjSpec* s, const mjVFS* vfs) {
 // attach body to a frame of the parent
 mjsBody* mjs_attachBody(mjsFrame* parent, const mjsBody* child,
                         const char* prefix, const char* suffix) {
+  if (!parent) {
+    mju_error("parent frame is null");
+    return nullptr;
+  }
   mjCFrame* frame_parent = static_cast<mjCFrame*>(parent->element);
   mjCBody* child_body = static_cast<mjCBody*>(child->element);
   try {
@@ -137,6 +141,10 @@ mjsBody* mjs_attachBody(mjsFrame* parent, const mjsBody* child,
 // attach frame to a parent body
 mjsFrame* mjs_attachFrame(mjsBody* parent, const mjsFrame* child,
                           const char* prefix, const char* suffix) {
+  if (!parent) {
+    mju_error("parent body is null");
+    return nullptr;
+  }
   mjCBody* body_parent = static_cast<mjCBody*>(parent->element);
   mjCFrame* child_frame = static_cast<mjCFrame*>(child->element);
   try {

src/user/user_objects.cc

@@ -802,7 +802,8 @@ mjCBody& mjCBody::operator=(const mjCBody& other) {
     sites.clear();
     cameras.clear();
     lights.clear();
-    id = other.id;
+    id = -1;
+    subtreedofs = 0;
 
     // add elements to lists
     *this += other;
@@ -1040,6 +1041,10 @@ void mjCBody::NameSpace_(const mjCModel* m, bool propagate) {
   for (auto& light : lights) {
     light->NameSpace(m);
   }
+
+  for (auto& frame : frames) {
+    frame->NameSpace(m);
+  }
 }