Add "frame" attribute to <attach> element.

The `<attach>` element now supports a "frame" attribute, allowing users to specify either a "body" or a "frame" to attach to, but not both. The XML parser has been updated to handle this new attribute and the mutual exclusivity constraint.

PiperOrigin-RevId: 941245904
Change-Id: I4c0edeed5f3a9e456aed96c373b1647fe79841d9

test/xml/testdata/child.xml

@@ -1,7 +1,18 @@
 <mujoco model="child">
   <worldbody>
     <body name="body">
-      <geom name="geom" size="1" pos="2 0 0"/>
+      <geom name="geom" size="1" pos="0 0 -2" rgba="1 0 0 1"/>
     </body>
+
+    <frame name="frame">
+      <body name="body2" pos="-1 0 0">
+        <freejoint/>
+        <geom name="geom2" type="box" size="1 1 1" rgba="0 1 0 1"/>
+      </body>
+      <body name="body3" pos="1 0 0">
+        <freejoint/>
+        <geom name="geom3" type="box" size="1 1 1" rgba="0 1 0 1"/>
+      </body>
+    </frame>
   </worldbody>
 </mujoco>

test/xml/testdata/parent.xml

@@ -3,9 +3,23 @@
     <model name="other" file="child.xml"/>
   </asset>
   <worldbody>
+    <!-- attach body to body -->
     <body name="parent">
-      <geom name="geom" size="2"/>
-      <attach model="other" body="body" prefix="other"/>
+      <geom name="geom" size="1" pos="0 0 2"/>
+      <attach model="other" body="body" prefix="other1"/>
     </body>
+
+    <!-- attach frame to body (world) -->
+    <attach model="other" frame="frame" prefix="other2"/>
+
+    <!-- attach frame to frame -->
+    <frame name="top" pos="0 0 4">
+      <attach model="other" frame="frame" prefix="other3"/>
+    </frame>
+
+    <!-- attach body to frame -->
+    <frame name="bottom-right" pos="2 0 -1">
+      <attach model="other" body="body" prefix="other4"/>
+    </frame>
   </worldbody>
 </mujoco>