Fix independent random perturbations

[saullocastro]

Summary

• ensure rdm_x and rdm_y are generated independently in piston theory tests

Testing

• pytest tests/test_quad4_piston_theory.py::test_quad4r_piston_theory -q (fails: ModuleNotFoundError: No module named 'pyfe3d.shellprop_utils')
• pip install numpy scipy cython (fails to complete due to network restrictions)

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https://chatgpt.com/codex/tasks/task_e_685578e07ce0832093a05e79057bde54

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (af9d2be) to head (3884ec6).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   99.93%   99.92%   -0.01%     
==========================================
  Files          11       11              
  Lines       37543    37543              
==========================================
- Hits        37517    37514       -3     
- Misses         26       29       +3     

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