Add Coveralls coverage reporting

[harryswift01]

Summary

This PR adds Coveralls coverage reporting to the existing prepmd CI workflow.

The test suite now runs with coverage enabled using pytest-cov, and coverage reports are uploaded to Coveralls as part of the GitHub Actions workflow.

Changes

Add coverage collection to CI:

• Installed pytest-cov as part of the CI test dependencies.
• Updated the test command to run with coverage enabled for the prepmd package.
• Added terminal coverage reporting with missing lines shown.
• Generated an lcov coverage report for upload.

Add Coveralls reporting:

• Added the Coveralls GitHub Action to upload coverage results.
• Configured coverage uploads for the CI matrix.
• Added a final Coveralls completion job for parallel matrix builds.

Impact

• Improves visibility of test coverage in the project.
• Makes it easier to monitor coverage changes over time.
• Helps identify areas of the codebase that may need additional tests.
• Does not change package functionality or runtime behaviour.