Add Jdot correction for connect and weld constaints

Measured reduction in constraint violations before/after this change:

| Model | Correction ON (Avg Viol) | Correction OFF (Avg Viol) | Reduction |
| :--- | :--- | :--- | :--- |
| `jdotv_connect_2d.xml` | 3.959e-4 | 1.699e-3 | **76.7%** |
| `jdotv_connect_3d.xml` | 1.399e-3 | 5.493e-3 | **74.5%** |
| `jdotv_weld_3d.xml` | 9.472e-3 | 1.148e-2 | **17.5%** |

PiperOrigin-RevId: 899137525
Change-Id: Ic3e33764ebd64239bab916289c23d80c3da0b51b

mjx/mujoco/mjx/_src/constraint_test.py

@@ -60,6 +60,9 @@ class ConstraintTest(parameterized.TestCase):
     # sample a mix of active/inactive constraints at different timesteps
     for key in range(3):
       mujoco.mj_resetDataKeyframe(m, d, key)
+      # scale down velocities to minimize Jdotv effect (not in MJX)
+      # TODO(team): remove this change when mjx supports this feature
+      d.qvel[:] *= 1e-2
       if rand_eq_active:
         d.eq_active[:] = np.random.randint(0, 2, size=m.neq)
       mujoco.mj_forward(m, d)

mjx/mujoco/mjx/_src/forward_test.py

@@ -49,6 +49,9 @@ class ForwardTest(absltest.TestCase):
     d.xfrc_applied[0, 2] = 0.1  # torque
     d.xfrc_applied[1, 4] = 0.3  # linear force
     mujoco.mj_step(m, d, 20)  # get some dynamics going
+    # scale down velocities to minimize Jdotv effect (not in MJX)
+    # TODO(team): remove this change when mjx supports this feature
+    d.qvel[:] *= 1e-2
     mujoco.mj_forward(m, d)
 
     mx = mjx.put_model(m)
@@ -92,6 +95,9 @@ class ForwardTest(absltest.TestCase):
     d.xfrc_applied[0, 2] = 0.1  # torque
     d.xfrc_applied[1, 4] = 0.3  # linear force
     mujoco.mj_step(m, d, 20)  # get some dynamics going
+    # scale down velocities to minimize Jdotv effect (not in MJX)
+    # TODO(team): remove this change when mjx supports this feature
+    d.qvel[:] *= 1e-2
 
     dx = jax.jit(mjx.step)(mjx.put_model(m), mjx.put_data(m, d))
     mujoco.mj_step(m, d)