Fix implicit integrator derivatives when actuator force is clamped.

The derivatives of actuator velocity with respect to generalized velocities were incorrectly computed when the actuator force was limited by `forcerange`. This CL adds a check to zero out these derivatives when the actuator force is at its upper or lower limit, as the force is no longer a function of velocity in this clamped state.

PiperOrigin-RevId: 868818281
Change-Id: I89742a3f04989dd11f9cfada107200015b1472a7

src/engine/engine_derivative.c

@@ -1089,6 +1089,15 @@ void mjd_actuator_vel(const mjModel* m, mjData* d) {
       continue;
     }
 
+    // skip if force is clamped by forcerange
+    if (m->actuator_forcelimited[i]) {
+      mjtNum force = d->actuator_force[i];
+      mjtNum* range = m->actuator_forcerange + 2*i;
+      if (force <= range[0] || force >= range[1]) {
+        continue;
+      }
+    }
+
     mjtNum bias_vel = 0, gain_vel = 0;
 
     // affine bias