Add mj_copyState

PiperOrigin-RevId: 836588419
Change-Id: I7609e121dc0ac697d4d015d4244bdd5962650def

python/mujoco/bindings_test.py

@@ -806,6 +806,13 @@ class MuJoCoBindingsTest(parameterized.TestCase):
     # Expect next states to be equal.
     np.testing.assert_array_equal(state1a, state1b)
 
+    # Test mj_copyState
+    data2 = mujoco.MjData(self.model)
+    mujoco.mj_copyState(self.model, self.data, data2, sig)
+    state1c = np.empty(size, np.float64)
+    mujoco.mj_getState(self.model, data2, state1c, sig)
+    np.testing.assert_array_equal(state1a, state1c)
+
   def test_mj_setKeyframe(self):  # pylint: disable=invalid-name
     mujoco.mj_step(self.model, self.data)