Refactor TF coil models to remove internal cross-section parameters a…

[chris-ashe]

This pull request refactors how the Central Solenoid (CS) poloidal cross-sectional area is handled in the TF coil stress calculations. Instead of calculating the CS area inside the stresscl function using several geometric parameters, the area is now passed directly as a parameter (a_cs_poloidal). This change simplifies the interface and removes the need for the previously used parameters. The refactor is applied across the main code and the unit tests.

Refactoring of CS poloidal area handling:

• The stresscl function in process/models/tfcoil/base.py now takes a_cs_poloidal as a direct argument and no longer requires z_tf_inside_half or f_z_cs_tf_internal. The area calculation inside the function has been removed, and all references to the removed parameters are deleted.

• All calls to stresscl in process/models/tfcoil/resistive.py and process/models/tfcoil/superconducting.py are updated to pass a_cs_poloidal instead of the removed parameters.
  Test updates:

• The StressclParam test data structure in tests/unit/models/tfcoil/test_tfcoil.py is updated to remove z_tf_inside_half and f_z_cs_tf_internal, and to add a_cs_poloidal. All test cases and calls to stresscl are updated accordingly.

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 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.78%. Comparing base (23787f8) to head (5276ca8).

Files with missing lines	Patch %	Lines
process/models/tfcoil/base.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4371   +/-   ##
=======================================
  Coverage   48.77%   48.78%           
=======================================
  Files         151      151           
  Lines       29390    29387    -3     
=======================================
  Hits        14336    14336           
+ Misses      15054    15051    -3     

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