Copy arena contents in mj_copyData.

Fixes #1710.

PiperOrigin-RevId: 651351384
Change-Id: I56e3204b30bb18f48278117e85e90464177da973

python/mujoco/bindings_test.py

@@ -418,6 +418,9 @@ class MuJoCoBindingsTest(parameterized.TestCase):
     data_copy = copy.copy(self.data)
     self.assertEqual(data_copy.ncon, 2)
 
+    # Make sure contact details are copied.
+    self.assertEqual(data_copy.contact[0].dist, self.data.contact[0].dist)
+
     # Make sure it's a copy.
     mujoco.mj_resetData(self.model, self.data)
     mujoco.mj_forward(self.model, self.data)