Add PFLocationTypes enum for PF coil locations

[chris-ashe]

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 71.42857% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.85%. Comparing base (ef00c94) to head (59df514).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/pfcoil.py	78.26%	5 Missing ⚠️
process/core/init.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4278      +/-   ##
==========================================
+ Coverage   48.83%   48.85%   +0.01%     
==========================================
  Files         151      151              
  Lines       29127    29138      +11     
==========================================
+ Hits        14223    14234      +11     
  Misses      14904    14904              

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[Copilot]

Pull request overview

This PR introduces a typed IntEnum for PF coil location modes and updates PF coil logic and initialization to use the enum instead of magic integers. It also includes small refactors to reuse existing derived variables for CS flux/geometry calculations, with corresponding test and documentation updates.

Changes:

• Add PFLocationTypes(IntEnum) and replace i_pf_location == {1,2,3,4} checks/assignments with enum members.
• Refactor CS-related calculations to use existing aggregated variables (vs_plasma_ramp_required, dz_cs_full, a_cs_poloidal) rather than recomputing equivalent expressions.
• Update a PF coil unit test and the PF coil documentation to reflect the new enum/type references.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
process/models/pfcoil.py	Adds PFLocationTypes and replaces numeric i_pf_location comparisons; refactors CS flux/geometry usage and CS filament current calculation.
process/core/init.py	Uses PFLocationTypes when setting/checking PF coil location defaults.
tests/unit/models/test_pfcoil.py	Adds test mocking for a_cs_poloidal to match the updated CS filament current calculation.
documentation/source/eng-models/pf-coil.md	Updates the “Positioning” section header to reference the enum/type.

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