Run Coveralls coverage on pull requests and main branch updates

[harryswift01]

Summary

This PR updates the pull request CI workflow so that full CI continues to run for pull requests, while coverage is also run on pushes to main to keep the Coveralls base branch coverage up to date.

Changes

Update GitHub Actions coverage workflow:

• Keep the workflow triggered for all pull requests.
• Keep the workflow triggered on pushes to main.
• Restrict the main CI jobs, including unit tests, regression tests, documentation, and pre-commit checks, to pull request events only.
• Allow the coverage job to run on both pull requests and pushes to main.
• Remove the coverage job dependency on the unit-test matrix so that coverage can still run when only the main push event is triggered.
• Add explicit workflow permissions for reading repository contents and publishing checks/PR coverage information.

Impact

• Full CI remains active for pull requests across all target branches.
• Pushes to main no longer rerun the full test suite unnecessarily.
• Coveralls receives updated coverage data from main, ensuring PR coverage comparisons use the latest base branch.
• Coverage differences in pull requests should be reported more consistently and reliably.

[jimboid]

More efficient CI pipeline usage is always a good thing.