Add tendon velocity sensor to MJX.

PiperOrigin-RevId: 685689974
Change-Id: Ib5e65b19c20b74e20cffa66c6baca0d8faaaf3ae

mjx/mujoco/mjx/test_data/sensor/sensor.xml

@@ -19,6 +19,7 @@
 -velocimeter
 -gyro
 -jointvel
+-tendonvel
 -actuatorvel
 -ballangvel
 -framelinvel
@@ -122,6 +123,7 @@
   <sensor>
     <torque site="site_torque"/>
     <tendonpos tendon="fixed"/>
+    <tendonvel tendon="fixed"/>
     <magnetometer name="magnetometer0" site="site0"/>
     <velocimeter name="velocimeter0" site="site0"/>
     <velocimeter name="velocimeter0cutoff" site="site0" cutoff="3e-4"/>
@@ -131,6 +133,7 @@
     <accelerometer name="accelerometer1" site="site1"/>
     <force site="site_force"/>
     <tendonpos tendon="spatial"/>
+    <tendonvel tendon="spatial"/>
     <jointpos name="jointpos0" joint="hinge0"/>
     <jointpos name="jointpos0cutoff" joint="hinge0" cutoff="1e-4"/>
     <jointvel name="jointvel0" joint="hinge0"/>