Fix bug in xml_native_writer that was causing frames to be skipped.

PiperOrigin-RevId: 683721505
Change-Id: I8622f4ea42ecda934fbc805c429de40e7717fdef

src/xml/xml_native_writer.cc

@@ -1718,11 +1718,6 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body, mjCFrame* frame, string_vi
                                : body->classname;
         Body(OneFrame(elem, fframe), body, fframe, childclass);
       }
-
-      // stop if we reached the frame of the current child body, ignore if there are no bodies
-      if (bframe && bframe == fframe) {
-        break;
-      }
     }
 
     // if there are no bodies, we only want to run the loop once

test/xml/xml_native_writer_test.cc

@@ -749,6 +749,9 @@ TEST_F(XMLWriterTest, WritesFrameDefaults) {
             </frame>
           </frame>
         </frame>
+        <frame>
+          <light pos="0 0 1"/>
+        </frame>
       </body>
     </worldbody>
   </mujoco>
@@ -775,6 +778,9 @@ TEST_F(XMLWriterTest, WritesFrameDefaults) {
           </frame>
         </frame>
       </frame>
+      <frame>
+        <light pos="0 0 1" dir="0 0 -1"/>
+      </frame>
     </body>
     <frame name="f1">
       <geom size="0.5" quat="0.906308 0 0 0.422618"/>