Upgrade to MacOS 15

[AlexanderWells-diamond]

MacOS 13 has been deprecated. See https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.16%. Comparing base (e817f12) to head (3a7f0dc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #195   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files          15       15           
  Lines        1259     1259           
=======================================
  Hits         1110     1110           
  Misses        149      149           

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