fix: update codecov-action to v6.0.2 to fix GPG verification failure

v6.0.0 (57e3a136) has a GPG signature verification issue when
downloading the codecov CLI on Ubuntu runners. v6.0.2 (fb8b3582)
released 2026-06-07 resolves this. Also keeps the 'files' input
fix from the previous commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: farhan

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
 
     - name: Upload coverage to CodeCov
       if: matrix.python-version == '3.12' && matrix.toxenv == 'django42'
-      uses: codecov/codecov-action@57e3a136b779b570ffcdbf80b3bdc90e7fab3de2 # v6.0.0
+      uses: codecov/codecov-action@fb8b3582c8e4def4969c97caa2f19720cb33a72f # v6.0.2
       with:
         file: ./coverage.xml
         fail_ci_if_error: true