Allow armature and damping on actuator elements

[adityasingh2400]

MuJoCo accepts armature and damping on the general, motor, position, velocity, intvelocity, damper, cylinder and muscle actuators, and the <default> section of schema.xml already listed both for exactly those eight elements. The <actuator> elements themselves did not, so the attributes could only be reached through a default class and setting either one directly on an actuator raised AttributeError: 'armature' is not a valid attribute for <position>.

I found this by probing every attribute name in schema.xml against every element with MuJoCo 3.11.0 and keeping the ones MuJoCo parses but PyMJCF rejects. armature and damping on those eight actuators were the only remaining gap. I also confirmed that adhesion correctly rejects both, so it is left alone, and that the parsed values reach actuator_armature and actuator_damping in the compiled model. The declarations mirror the ones already used in the <default> block.

The new tests cover all eight actuator types and a round trip through MjModel that asserts the values survive. All nine fail before this change and pass after it.

[adityasingh2400]

Closing this as superseded by 985d094, which replaced the hand-maintained schema.xml with the generated schema from MuJoCo's mjcf.schema.

I verified the specific gap this PR filled: the <general> actuator element on current main now carries both armature and damping, so the schema no longer rejects them and the hand edit here is redundant.