Add support for indexing qfrc fields in MJX bind() method.

PiperOrigin-RevId: 763415415
Change-Id: Idb567a1711679565444779fb1062d7b65434ef13

mjx/mujoco/mjx/_src/support_test.py

@@ -249,6 +249,11 @@ class SupportTest(parameterized.TestCase):
           dx.bind(mx, s.joints[i]).qacc,
           d.qacc[m.jnt_dofadr[i]:m.jnt_dofadr[i] + dofnum[i]], decimal=6
       )
+      np.testing.assert_array_almost_equal(
+          dx.bind(mx, s.joints[i]).qfrc_actuator,
+          d.qfrc_actuator[m.jnt_dofadr[i] : m.jnt_dofadr[i] + dofnum[i]],
+          decimal=6,
+      )
 
     np.testing.assert_array_equal(dx.bind(mx, s.actuators).ctrl, d.ctrl)
     for i in range(m.nu):
@@ -344,7 +349,7 @@ class SupportTest(parameterized.TestCase):
     ):
       print(dx.bind(mx, s.actuators).set('actuator_ctrl', [1, 2, 3]))
     with self.assertRaisesRegex(
-        AttributeError, 'qpos, qvel, qacc are not available for this type'
+        AttributeError, 'qpos, qvel, qacc, qfrc are not available for this type'
     ):
       print(dx.bind(mx, s.geoms).qpos)