Improve robustness of t_zero_margin calculation with brentq fallback

[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 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.06%. Comparing base (2f754a2) to head (0b1e28f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
process/superconducting_tf_coil.py	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4018      +/-   ##
==========================================
- Coverage   45.88%   44.06%   -1.82%     
==========================================
  Files         123      123              
  Lines       29087    30955    +1868     
==========================================
+ Hits        13346    13641     +295     
- Misses      15741    17314    +1573     

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