ci: add Coveralls for code coverage reporting

- Add Coveralls GitHub Action to upload coverage reports
- Configure pytest to generate coverage reports in XML format
- Add term-missing report for better visibility in CI logs
- Use built-in GITHUB_TOKEN for authentication

This will provide code coverage feedback directly in pull requests
and track coverage trends over time.

.github/workflows/test-code.yml

@@ -38,4 +38,9 @@ jobs:
 
       - name: "Run tests"
         run: |
-          pytest --cov=src --cov-report=xml
+          pytest --cov=src --cov-report=xml --cov-report=term-missing
+
+      - name: Upload coverage to Coveralls
+        uses: coverallsapp/github-action@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}