Add Coveralls Coverage Reporting to CI

[harryswift01]

Currently, the prepmd CI workflow does not report test coverage. To improve visibility of test coverage and align with other CCPBioSim repositories, I propose integrating Coveralls into the existing CI workflow.

This would allow coverage results to be reported automatically when tests are run in CI.

Suggested changes:

• Add coverage collection to the test workflow.
• Add Coveralls integration to the CI workflow.
• Configure the repository to upload coverage results to Coveralls.

This would make it easier to monitor test coverage over time and help identify areas of the codebase that may need additional tests.