Add Codecov integration for coverage reporting

- Generate XML coverage report for Codecov
- Upload coverage using codecov-action@v4
- Requires CODECOV_TOKEN secret in repository settings

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Marat Saidov

.github/workflows/ci.yml

@@ -30,4 +30,12 @@ jobs:
 
       - name: Run tests with coverage
         run: |
-          python -m pytest tests/ -v --tb=short --cov=src --cov-report=term-missing
+          python -m pytest tests/ -v --tb=short --cov=src --cov-report=term-missing --cov-report=xml
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v4
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: ./coverage.xml
+          fail_ci_if_error: false
+          verbose: true