test(cron): the run_job FakeFuture double answers done() like the real Future

run_job's finally now asks the worker Future whether it is still running before deciding who tears the session down; the heartbeat test's stand-in future lacked done().

tests/cron/test_scheduler.py

@@ -921,6 +921,9 @@ class TestRunJobSessionPersistence:
             def result(self):
                 return {"final_response": "ok"}
 
+            def done(self):
+                return True  # run_job's finally asks the real Future; this one has already returned
+
         fake_future = FakeFuture()
         fake_pool = MagicMock()
         fake_pool.submit.return_value = fake_future