Add pytest coverage reporting to CI pipeline

[armalikhani]

Add pytest coverage reporting to CI pipeline

Changes:

• Added pytest-cov dependency
• Updated CI test step to run coverage with pytest
• Uploads coverage.xml as a CI artifact for inspection anf use
• Coverage output shown in CI logs

Future tasks:

• Consider integrating Codecov to upload coverage reports
• Optionally enforce a minimum coverage threshold in CI
• Add a coverage badge to the README once Codecov is configured

Closes #28