fix(tests): remove dead step_assert_no_deadlocks and fix ruff formatting

Removed the unreachable step_assert_no_deadlocks function from
invariant_service_thread_safety_mixed_steps.py. The step was never
referenced by any Gherkin scenario and contained a broken reference
to context.elapsed_time which is never set, causing an AttributeError
if the step were ever invoked. Applied ruff auto-formatting to fix
trailing blank lines left after the removal.

ISSUES CLOSED: #7524

features/steps/invariant_service_thread_safety_mixed_steps.py

@@ -219,12 +219,3 @@ def step_assert_results_consistent(context: Any) -> None:
     assert len(context.errors) == 0, (
         f"Expected no errors, got {len(context.errors)}: {context.errors}"
     )
-
-
-@then("no deadlocks should occur")
-def step_assert_no_deadlocks(context: Any) -> None:
-    """Assert that no deadlocks occurred by verifying the test completed in time."""
-    assert context.elapsed_time < 30, (
-        f"Test took {context.elapsed_time:.2f}s — possible deadlock or starvation"
-    )
-