ci: upload coverage to Codecov

Switch to codecov-action@v5 and explicitly upload coverage.xml.

Made-with: Cursor

Author: Sayeem3051

.github/workflows/ci.yml

@@ -52,9 +52,10 @@ jobs:
           pytest tests/ --cov=ctxeng --cov-report=xml --cov-report=term-missing
 
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v4
+        uses: codecov/codecov-action@v5
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
+          files: ./coverage.xml
           fail_ci_if_error: false
 
   publish: