test: make payload failure regression return explicit

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 064f1284-f637-4ba9-8406-8c094ce9f845

Author: andystaples

tests/durabletask/test_orchestrator_payload_failure.py

@@ -163,6 +163,7 @@ def orchestrator(ctx, _):
         if mode == "status":
             ctx.set_custom_status(LARGE_PAYLOAD)
         yield ctx.call_activity("after_timeout", input=LARGE_PAYLOAD if mode == "activity" else None)
+        return None
 
     store = FakePayloadStore(threshold_bytes=100, max_stored_payload_bytes=200)
     stub = MagicMock()