ci: various workflow improvements

[erisu]

Platforms affected

n/a

Motivation and Context

Use macos-15 in CI

Description

• Replaced macos-latest with macos-15
• Added missing permissions block
• Pinned Codecov

Testing

n/a

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.68%. Comparing base (a8f7483) to head (6351d6c).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #670   +/-   ##
=======================================
  Coverage   72.68%   72.68%           
=======================================
  Files           3        3           
  Lines         615      615           
=======================================
  Hits          447      447           
  Misses        168      168           

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[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.