chore: drop the blank lines the rebase left, so black --check passes

lint is a CI gate at black==24.2.0; the rebase onto master left three
blank lines before _StubAgent. No behaviour change.

Author: ayaangazali

tests/structs/test_graph_workflow.py

@@ -1483,8 +1483,6 @@ def test_save_spec_round_trips_through_from_topology_spec(tmp_path):
     pytest.main([__file__, "-v"])
 
 
-
-
 class _StubAgent:
     """Agent stand-in that records calls and returns a canned string."""