chore(ci): prevent test log truncation in backend coverage workflows

- Install gotestsum in CI so the coverage script uses compact
  pkgname-formatted output instead of go test -v, which produces
  massive verbose logs that exceed GitHub Actions' step log buffer
- Upload the full test output as a downloadable artifact on every
  run (including failures) so truncated logs never block debugging
- Aligns upload-artifact pin to v7.0.0 matching the rest of the repo

.github/workflows/codecov-upload.yml

@@ -126,6 +126,9 @@ jobs:
             echo "__CHARON_EOF__"
           } >> "$GITHUB_ENV"
 
+      - name: Install gotestsum
+        run: go install gotest.tools/gotestsum@latest
+
       - name: Run Go tests with coverage
         working-directory: ${{ github.workspace }}
         env:
@@ -134,6 +137,14 @@ jobs:
           bash scripts/go-test-coverage.sh 2>&1 | tee backend/test-output.txt
           exit "${PIPESTATUS[0]}"
 
+      - name: Upload test output artifact
+        if: always()
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
+        with:
+          name: backend-test-output
+          path: backend/test-output.txt
+          retention-days: 7
+
       - name: Upload backend coverage to Codecov
         uses: codecov/codecov-action@1af58845a975a7985b0beb0cbe6fbbb71a41dbad # v5
         with: