Refactor plasma field calculations to use numba's njit for improved p…

[chris-ashe]

…erformance

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.

[Copilot]

Pull request overview

This PR updates plasma field helper functions to use Numba’s njit decorator (nopython mode) for improved JIT performance and stricter compilation guarantees within the physics model layer.

Changes:

• Replace @nb.jit(cache=True) with @nb.njit(cache=True) on several PlasmaFields static methods.
• Ensure the affected plasma field calculations run in Numba nopython mode (consistent with other physics modules already using njit / jit(nopython=True)).

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.44%. Comparing base (0a46aa3) to head (1400951).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4172   +/-   ##
=======================================
  Coverage   49.44%   49.44%           
=======================================
  Files         149      149           
  Lines       29796    29796           
=======================================
  Hits        14734    14734           
  Misses      15062    15062           

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