test: clean up git and artifact warnings (#1244)

* initialize the temporary Git repository in test_workflow_git with --initial-branch=main
* avoid Git's default-branch advisory in pytest logs without changing global Git configuration or suppressing stderr
* upload generated optimization result artifacts only when the test job fails
* use the actual nested tests/testdata/**/new_optimize_result* paths
* ignore the no-files case so unrelated test failures do not produce an artifact warning

.github/workflows/pytest.yml

@@ -35,7 +35,9 @@ jobs:
           uv run pytest --finalize --check-config-side-effect -vs --cov src --cov-report term-missing
 
       - name: Upload test artifacts
+        if: failure()
         uses: actions/upload-artifact@v7
         with:
           name: optimize-results
-          path: tests/testdata/new_optimize_result*
+          path: tests/testdata/**/new_optimize_result*
+          if-no-files-found: ignore