ci: re-add mac os unit tests on main

[kevinzwang]

Changes Made

Follow-up to #5142 to add the mac tests back for commits on the main branch.

Related Issues

Checklist

• Documented in API Docs (if applicable)
• Documented in User Guide (if applicable)
• If adding a new documentation page, doc is added to docs/mkdocs.yml navigation
• Documentation builds and is formatted properly (tag @/ccmao1130 for docs review)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.29%. Comparing base (33884be) to head (ed888f3).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5163      +/-   ##
==========================================
+ Coverage   73.81%   74.29%   +0.47%     
==========================================
  Files         957      957              
  Lines      124278   123246    -1032     
==========================================
- Hits        91740    91561     -179     
+ Misses      32538    31685     -853     

see 34 files with indirect coverage changes

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[greptile-apps]

Greptile Summary

This PR adds macOS unit tests back to the CI pipeline, but only for commits on the main branch rather than for pull requests. The change modifies the GitHub Actions workflow to use a matrix strategy that includes both Ubuntu and macOS runners, with strategic exclusions to prevent macOS tests from running on PRs.

The implementation works by adding an os matrix dimension with both ubuntu-latest and macos-latest, and introducing an on-main matrix variable that evaluates whether the current Git reference is the main branch (${{ github.ref == 'refs/heads/main' }}). The workflow then uses matrix exclusions to prevent macOS jobs from running when on-main: false, effectively limiting expensive macOS tests to main branch commits only.

Additionally, the PR conditionalizes Ubuntu-specific setup steps (like disk space cleanup and apt package installation) to only run on Linux runners using if: ${{ (runner.os == 'Linux') }} conditions. This change follows PR #5142, which originally removed macOS tests from PRs due to cost concerns, and represents a balanced approach to maintain platform coverage on the production branch while keeping PR testing efficient.

Confidence score: 4/5

• This PR is safe to merge with minimal risk as it only affects CI configuration without touching production code
• Score reflects straightforward GitHub Actions matrix configuration with well-tested conditional logic patterns
• Pay close attention to the matrix exclusion logic to ensure macOS tests actually run on main branch pushes

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