Rename codecov input file → files (renamed in codecov-action v6)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: chiliec

.github/workflows/tests.yml

@@ -33,4 +33,4 @@ jobs:
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v6
         with:
-          file: ./coverage.xml
+          files: ./coverage.xml