Enforce tresca mises methods in tf stresscl()

[chris-ashe]

This pull request refactors the calculation of Tresca and Von Mises stresses in the codebase to improve performance and maintainability. The main changes involve replacing legacy, hand-coded stress calculation logic with optimized, reusable functions, and updating all usages to rely on these new implementations. Additionally, related tests and unused code are cleaned up.

• Updated process/models/tfcoil/base.py to use the new calculate_tresca_stress and calculate_von_mises_stress functions throughout, replacing manual calculations for both array and scalar cases.

Code cleanup:

• Removed the old sigvm function from process/models/tfcoil/base.py as it is now superseded by the new stress calculation functions.
• Deleted the obsolete test_sigvm test from tests/unit/models/tfcoil/test_tfcoil.py that directly tested the removed sigvm function.

These changes make the stress calculations more efficient, easier to maintain, and consistent across the codebase.## Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.63%. Comparing base (616f749) to head (18e8c43).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/tfcoil/base.py	20.00%	4 Missing ⚠️
process/models/engineering/materials.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4433      +/-   ##
==========================================
- Coverage   48.63%   48.63%   -0.01%     
==========================================
  Files         151      151              
  Lines       29673    29674       +1     
==========================================
  Hits        14432    14432              
- Misses      15241    15242       +1     

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[timothy-nunn]

Happy with code changes. I have a few requests about tests to ensure these functions work with (combinations of) the types stated.

[timothy-nunn]

Could you write a couple of quick tests for calculate_tresca_stress that check it returns the expected value when running with floats, numpy arrays, and mixtures of the two (if this makes sense).

[timothy-nunn]

Is there an existing test for calculate_von_mises_stress? if not could this test be copied into the appropriate location for that function