Bound MQTT broker operations with a per-node timeout

Without one, aiomqtt's disconnect acknowledgement has no deadline, so a
subscriber cancelled while its socket is dead never finishes unwinding and
teardown abandons the task. The knob is per node because brokers differ.

backend/tests/flow/test_senders.py

@@ -89,3 +89,33 @@ def test_a_string_goes_on_the_wire_bare():
     asyncio.run(node._publish_with(client, {"plug": "ON", "level": 60}))
 
     assert client.published == [("actor/plug", "ON"), ("light/level", "60")]
+
+
+def test_the_configured_timeout_reaches_the_broker_client(monkeypatch):
+    """Without one, a dead socket makes the disconnect ack wait forever."""
+    import aiomqtt
+
+    seen: dict = {}
+
+    class FakeClient:
+        def __init__(self, **kwargs):
+            seen.update(kwargs)
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, *_):
+            return False
+
+        async def publish(self, *_, **__):
+            return None
+
+    monkeypatch.setattr(aiomqtt, "Client", FakeClient)
+
+    node = MqttNode(
+        requires=[MessageSpec(name="setpoint", port="setpoint", dtype=DType.FLOAT)],
+        params={"topic": {"setpoint": "heating/setpoint"}, "timeout": 2.5},
+    )
+    asyncio.run(node._publish_once({"setpoint": 21.0}))
+
+    assert seen["timeout"] == 2.5