ci: codecov-action@v5 renamed input file -> files

Clears the "Unexpected input(s) 'file'" annotation from the v4->v5 bump.
Codecov upload remains fail_ci_if_error: false (non-gating).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: RFingAdam

.github/workflows/tests.yml

@@ -49,7 +49,7 @@ jobs:
       if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'
       uses: codecov/codecov-action@v5
       with:
-        file: ./coverage.xml
+        files: ./coverage.xml
         flags: unittests
         name: codecov-${{ matrix.os }}-py${{ matrix.python-version }}
         fail_ci_if_error: false