Fix AMS slot changes failing until manual reconnect (#887)

Add timeout and retry to the developer mode probe. After a keep-alive
  timeout, paho auto-reconnects but the session can be half-broken: the
  printer sends status but ignores commands. The probe had no recovery —
  one unanswered probe permanently blocked retries. Now times out after
  10s with one retry; two consecutive failures force-close the socket for
  a clean reconnect.

backend/tests/unit/services/test_bambu_mqtt.py

@@ -2788,6 +2788,147 @@ class TestDeveloperModeDetection:
         assert mqtt_client.state.developer_mode is False
 
 
+class TestDeveloperModeProbeTimeout:
+    """Tests for developer mode probe timeout, retry, and forced reconnect (#887).
+
+    When a printer's MQTT session is half-broken (sends status but ignores
+    commands), the developer mode probe gets no response.  The timeout logic
+    retries once, then force-closes the socket on the second failure.
+    """
+
+    @pytest.fixture
+    def mqtt_client(self):
+        from unittest.mock import MagicMock
+
+        from backend.app.services.bambu_mqtt import BambuMQTTClient
+
+        client = BambuMQTTClient(
+            ip_address="192.168.1.100",
+            serial_number="TEST123",
+            access_code="12345678",
+        )
+        # Simulate connected state with a mock MQTT client
+        client.state.connected = True
+        mock_paho = MagicMock()
+        mock_paho.socket.return_value = MagicMock()
+        client._client = mock_paho
+        return client
+
+    def _make_pushall_data(self):
+        """Create a print data dict with >30 keys (triggers probe) and no 'fun' field."""
+        return {f"key_{i}": i for i in range(35)}
+
+    def test_first_timeout_allows_retry(self, mqtt_client):
+        """After first probe timeout, _dev_mode_probed resets to allow retry."""
+        import time
+
+        data = self._make_pushall_data()
+
+        # First pushall triggers the probe
+        mqtt_client._update_state(data)
+        assert mqtt_client._dev_mode_probed is True
+        assert mqtt_client._dev_mode_probe_seq is not None
+        assert mqtt_client.state.developer_mode is None
+
+        # Simulate 11 seconds passing
+        mqtt_client._dev_mode_probe_time = time.monotonic() - 11.0
+
+        # Next status message detects the timeout
+        mqtt_client._update_state(data)
+        assert mqtt_client._dev_mode_probe_failures == 1
+        assert mqtt_client._dev_mode_probe_seq is None
+        # Should allow retry on next full message
+        assert mqtt_client._dev_mode_probed is False
+        # Connection should NOT be force-closed after 1 failure
+        assert mqtt_client.state.connected is True
+
+    def test_second_timeout_forces_reconnect(self, mqtt_client):
+        """After two consecutive probe timeouts, force-close the socket."""
+        import time
+
+        data = self._make_pushall_data()
+        state_change_called = []
+        mqtt_client.on_state_change = lambda s: state_change_called.append(True)
+
+        # First probe + timeout
+        mqtt_client._update_state(data)
+        mqtt_client._dev_mode_probe_time = time.monotonic() - 11.0
+        mqtt_client._update_state(data)
+        assert mqtt_client._dev_mode_probe_failures == 1
+
+        # Second probe (retry) + timeout
+        mqtt_client._update_state(data)  # triggers new probe
+        assert mqtt_client._dev_mode_probed is True
+        mqtt_client._dev_mode_probe_time = time.monotonic() - 11.0
+        mqtt_client._update_state(data)  # detects second timeout
+
+        assert mqtt_client._dev_mode_probe_failures == 2
+        assert mqtt_client.state.connected is False
+        assert mqtt_client._stale_reconnecting is True
+        # Socket should have been closed
+        mqtt_client._client.socket().close.assert_called()
+        # on_state_change should have been called
+        assert len(state_change_called) > 0
+
+    def test_successful_probe_resets_failure_counter(self, mqtt_client):
+        """A probe response after a previous failure resets the counter."""
+        import time
+
+        data = self._make_pushall_data()
+
+        # First probe + timeout → failure=1
+        mqtt_client._update_state(data)
+        seq = mqtt_client._dev_mode_probe_seq
+        mqtt_client._dev_mode_probe_time = time.monotonic() - 11.0
+        mqtt_client._update_state(data)
+        assert mqtt_client._dev_mode_probe_failures == 1
+
+        # Retry probe
+        mqtt_client._update_state(data)
+        new_seq = mqtt_client._dev_mode_probe_seq
+        assert new_seq is not None
+        assert new_seq != seq
+
+        # Simulate successful response
+        mqtt_client._handle_dev_mode_probe_response(
+            {
+                "command": "ams_filament_setting",
+                "sequence_id": new_seq,
+                "result": "success",
+            }
+        )
+        assert mqtt_client._dev_mode_probe_failures == 0
+        assert mqtt_client.state.developer_mode is True
+        assert mqtt_client._dev_mode_probe_seq is None
+
+    def test_no_timeout_when_probe_not_sent(self, mqtt_client):
+        """The timeout branch is only entered when a probe is pending."""
+        # No probe sent — _dev_mode_probed is False, _dev_mode_probe_seq is None
+        data = {"gcode_state": "IDLE", "mc_percent": 0}  # < 30 keys
+        mqtt_client._update_state(data)
+        assert mqtt_client._dev_mode_probe_failures == 0
+
+    def test_on_connect_resets_probe_state(self, mqtt_client):
+        """_on_connect resets all probe-related state for fresh detection."""
+        import time
+
+        mqtt_client._dev_mode_probed = True
+        mqtt_client._dev_mode_probe_seq = "42"
+        mqtt_client._dev_mode_probe_time = time.monotonic()
+        mqtt_client._dev_mode_probe_failures = 2
+        mqtt_client.state.developer_mode = True
+
+        # subscribe() must return (result, mid) tuple
+        mqtt_client._client.subscribe.return_value = (0, 1)
+        mqtt_client._on_connect(mqtt_client._client, None, None, 0)
+
+        assert mqtt_client.state.developer_mode is None
+        assert mqtt_client._dev_mode_probed is False
+        assert mqtt_client._dev_mode_probe_seq is None
+        assert mqtt_client._dev_mode_probe_time == 0.0
+        assert mqtt_client._dev_mode_probe_failures == 0
+
+
 class TestVtTrayNormalization:
     """Tests for vt_tray dict→list normalization in _update_state.