Associate children of deleted body to the newly created frame in mjs_bodyToFrame.

Also associate all children that need a parent body to the parent of the deleted body.

This is a bug that was causing the former children of the body that gets transformed to a frame to not be children of the new frame.

PiperOrigin-RevId: 702702853
Change-Id: I36d48f0446bc87c665b18d34e0b0609d82c70f51

test/user/user_api_test.cc

@@ -1316,7 +1316,7 @@ TEST_F(MujocoTest, AttachWorld) {
   static constexpr char xml_parent[] = R"(
   <mujoco>
     <worldbody>
-      <frame name="frame"/>
+      <frame name="frame" pos="1 2 3"/>
     </worldbody>
   </mujoco>)";
 
@@ -1327,18 +1327,20 @@ TEST_F(MujocoTest, AttachWorld) {
         <joint type="slide"/>
         <geom size=".1"/>
       </body>
+      <camera pos="0 0 0" quat="1 0 0 0"/>
     </worldbody>
   </mujoco>)";
 
   static constexpr char xml_result[] = R"(
   <mujoco>
     <worldbody>
-      <frame name="frame">
+      <frame name="frame" pos="1 2 3">
         <frame name="attached-world-1">
           <body name="attached-sphere-1">
             <joint type="slide"/>
             <geom size=".1"/>
           </body>
+          <camera pos="0 0 0" quat="1 0 0 0"/>
         </frame>
       </frame>
     </worldbody>