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_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);
+  }
 }