ci: pass CODECOV_TOKEN explicitly to codecov-action (#21)

codecov-action@v4 was not picking up CODECOV_TOKEN from env
automatically in this workflow (log shows 'no token was provided'
even though the secret is configured in repo settings). Pass it
explicitly via the token input to make uploads reliable.

Also add:
- files: ./coverage.xml - explicit report path
- fail_ci_if_error: false - coverage upload failures shouldn't
  break the CI run

Author: maltsev-dev

.github/workflows/ci.yml

@@ -45,4 +45,8 @@ jobs:
       - run: pip install -e ".[dev]"
       - run: coverage run -m pytest
       - uses: codecov/codecov-action@v4
-        if: always()
+        if: always()
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: ./coverage.xml
+          fail_ci_if_error: false