Convert Newton angular attributes to MuJoCo units

[sylvesterkaczmarek]

Summary

Fix the two angular-unit conversion defects reported in #3472 for the Newton USD schema read path.

Newton authors newton:damping for angular degrees as effort-seconds per degree, while MuJoCo stores angular joint damping per radian. Newton also authors newton:mimicCoef0 in degrees for revolute joints, while MuJoCo's mjEQ_JOINT constant is in radians.

This change:

• multiplies Newton damping by 180/pi for angular MuJoCo joints
• converts newton:mimicCoef0 from degrees to radians for USD revolute joints
• leaves slide-joint and dimensionless values unchanged
• leaves the deprecated mjc: paths unchanged

Addresses parts 1 and 2 of #3472. The separate newton:mimicEnabled defect is already being handled by #3477.

Validation

• Based directly on current upstream main (80d1b0d81148e4f85b0260bc0d5f0ca92ca52113).
• Final branch contains one commit (99b71edb13b25905aa5938dfe6774b31147e35fb) and changes only plugin/usd_decoder/usd_decoder.cc.
• git diff --check passes.
• The OpenUSD decoder test environment was not built locally, so no runtime test pass is claimed.