Improvements related to models where joint-actuator relationship is not one-to-one:

- Add `joint-actuatorforcerange` for clamping total actuator force at joints. Add `sensor-jointactuatorfrc` for sensing total actuator forces on a single joint.
See [documentation](https://mujoco.readthedocs.io/en/latest//modeling.html#actuator-force-clamping) for justification and use cases.
- Add simple car model to `model/`.
- Move actuation-related test models into `engine/testdata/actuation/`.

PiperOrigin-RevId: 549355941
Change-Id: I27f6c1f80426d73a2811ef5ae74684a228b2fbd1

introspect/structs.py

@@ -1391,6 +1391,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='does joint have limits                   (njnt x 1)',
              ),
+             StructFieldDecl(
+                 name='jnt_actfrclimited',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtByte'),
+                 ),
+                 doc='does joint have actuator force limits    (njnt x 1)',
+             ),
              StructFieldDecl(
                  name='jnt_solref',
                  type=PointerType(
@@ -1433,6 +1440,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='joint limits                             (njnt x 2)',
              ),
+             StructFieldDecl(
+                 name='jnt_actfrcrange',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum'),
+                 ),
+                 doc='range of total actuator force            (njnt x 2)',
+             ),
              StructFieldDecl(
                  name='jnt_margin',
                  type=PointerType(