ci(github): enable artifact merging

Closes #341

Thanks to @muzimuzhi for the tip!

.github/workflows/server-tests.yml

@@ -64,6 +64,7 @@ jobs:
         uses: actions/download-artifact@v4
         with:
           path: ${{ github.workspace }}/server/
+          merge-multiple: true
       - name: Combine coverage reports and generate Cobertura XML report
         run: |
           coverage combine