Create bootstrap current class

[chris-ashe]

This pull request refactors how the plasma bootstrap current calculations are integrated and tested in the codebase. The main improvement is the explicit use and injection of the PlasmaBootstrapCurrent class throughout the main process and test modules, rather than relying on methods attached to the Physics class. This enhances clarity, modularity, and testability of bootstrap current calculations.

Core integration and dependency injection:

• The PlasmaBootstrapCurrent class is now explicitly instantiated and injected as a dependency into the main process (process/main.py) and the Stellarator model, ensuring that bootstrap current calculations are managed as a separate, dedicated component.

Test suite refactoring:

• All unit tests for bootstrap current fractions in tests/unit/test_physics.py have been updated to directly instantiate and use PlasmaBootstrapCurrent, removing reliance on the Physics class for these calculations. This clarifies test intent and decouples bootstrap current tests from unrelated physics logic.

Stellarator model and test updates:

• The Stellarator model and its corresponding test fixture now accept and use the PlasmaBootstrapCurrent class as a parameter, maintaining consistency with the main process and supporting improved modularity in the stellarator workflow.

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 68.05556% with 92 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.09%. Comparing base (4741c15) to head (4f9aa32).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/physics/bootstrap_current.py	68.07%	91 Missing ⚠️
process/main.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4091      +/-   ##
==========================================
+ Coverage   46.88%   47.09%   +0.21%     
==========================================
  Files         136      137       +1     
  Lines       29253    29290      +37     
==========================================
+ Hits        13715    13794      +79     
+ Misses      15538    15496      -42     

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