`MjSpec.{option,statistic,visual}`: "Unable to convert function return value to a Python type!"

Intro

Hi!

I use MuJoCo for robotic manipulation.

My setup

MuJoCo:

$ python -c "import mujoco; print(mujoco.__version__)"
3.2.3

API: Python

OS:

$ python -c "import platform; print(f'{platform.system()=}, {platform.release()=}, {platform.machine()=}')"
platform.system()='Darwin', platform.release()='24.0.0', platform.machine()='arm64'

What's happening? What did you expect?

The title pretty much describes the problem: the option, statistic, or visual attributes of MjSpec are inaccessible because of the following error:

python -c "import mujoco; mujoco.MjSpec().visual"
TypeError: Unregistered type : mjVisual_

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: Unable to convert function return value to a Python type! The signature was
        (arg0: mujoco._specs.MjSpec) -> mjVisual_

Steps for reproduction

Add the following to specs_test.py and run the test:

  def test_access_option_stat_visual(self):
    spec = mujoco.MjSpec()
    spec.from_string(textwrap.dedent("""
      <mujoco model="MuJoCo Model">
        <option timestep="0.001"/>
        <statistic meansize="0.05"/>
        <visual>
          <quality shadowsize="4096"/>
        </visual>
      </mujoco>
    """))
    self.assertEqual(spec.option.timestep, 0.001)
    self.assertEqual(spec.statistic.meansize, 0.05)
    self.assertEqual(spec.visual.quality.shadowsize, 4096)

    spec.option.timestep = 0.002
    spec.statistic.meansize = 0.06
    spec.visual.quality.shadowsize = 8192

    model = spec.compile()

    self.assertEqual(model.opt.timestep, 0.002)
    self.assertEqual(model.stat.meansize, 0.06)
    self.assertEqual(model.vis.quality.shadowsize, 8192)

Minimal model for reproduction

See above.

Code required for reproduction