Add URDF transmission coupling and tests

[jslee02]

Summary

• add ParseContext to keep URDF transmission parsing scalable
• wire SimpleTransmission parsing into mimic/coupler constraints
• add integration tests for transmission dynamics and invalid cases
• document mimic/coupler usage

Testing

• pixi run test

Issues

Fixes #1047

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (57064be) to head (44e34b6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #2281       +/-   ##
==========================================
- Coverage   61.82%       0   -61.83%     
==========================================
  Files         360       0      -360     
  Lines       32592       0    -32592     
  Branches     4283       0     -4283     
==========================================
- Hits        20149       0    -20149     
+ Misses      12443       0    -12443     

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