ci(deps): Bump codecov/codecov-action from v5 to v6

why: GitHub Actions will require Node.js 24 by June 2, 2026.
codecov-action v6 adds Node 24 support with no config changes needed.
what:
- Update codecov/codecov-action@v5 → @v6 in tests.yml
- No other changes required (v6 is functionally identical to v5)
- Ref: https://github.com/codecov/codecov-action/releases/tag/v6.0.0

Author: tony

.github/workflows/tests.yml

@@ -42,7 +42,7 @@ jobs:
       - name: Test with pytest
         run: uv run py.test --cov=./ --cov-report=xml
 
-      - uses: codecov/codecov-action@v5
+      - uses: codecov/codecov-action@v6
         with:
           token: ${{ secrets.CODECOV_TOKEN }}