Add Codecov coverage reporting to CI pipeline

[EduardoRasanmar]

Summary

Integrate Codecov into the CI pipeline to publish test coverage results and display coverage badges for the project.

This addresses the course requirement to generate a coverage report in CI and upload it to a coverage service.

---

Motivation

• Make test coverage visible and verifiable
• Support CI-based quality checks
• Align with DSCI 524 project requirements (coverage + badge)

---

Tasks

• Ensure pytest runs with coverage enabled
• Generate coverage.xml in CI (pytest --cov --cov-report=xml --cov-branch)
• Add Codecov upload step using codecov/codecov-action
• Configure repository upload token via GitHub Secrets
• Verify successful upload in CI
• Add Codecov badge to README (after merge)

---

Related Work

• Branch: fix/codecov-coverage