Add named element accessors to mjx.Model

[Shivam-Bhardwaj]

Summary

Add convenience methods to mjx.Model for accessing elements by name, similar to mujoco.MjModel.

Closes #2806

Changes

• Add BodyView, JointView, GeomView, SiteView, ActuatorView dataclasses
• Add body(), joint(), geom(), site(), actuator() methods to Model
• Add unit tests for all accessor methods

Example usage

import mujoco
from mujoco import mjx

model = mujoco.MjModel.from_xml_string(xml)
modelx = mjx.put_model(model)

# Access elements by name (similar to MjModel)
body = modelx.body("body1")
print(body.id)    # same as model.body("body1").id
print(body.name)  # "body1"

joint = modelx.joint("joint1")
geom = modelx.geom("geom1")
actuator = modelx.actuator("motor1")

Test plan

• Added unit tests comparing results with mujoco.MjModel accessors
• Tests verify KeyError is raised for non-existent elements
• Ran pyink and isort for style compliance