add macos-26 to CI runs

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

add macos-26 to CI runs

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (137133c) to head (1e65e8b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1164   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          14       14           
  Lines        5924     5924           
=======================================
  Hits         5382     5382           
  Misses        542      542           

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[jgabry]

We could potentially remove this when macOS-latest bumps to 26