fix(drying): stop false "drying complete" from killing the printer via smart-plug auto-off (#1462)

Reporter on X2D set a 1h AMS dry; the printer powered off seconds in.
  Support log: every "Sent drying command duration=1" was followed 3-9s
  later by "AMS 0 drying complete (dry_time 60 -> 0)" -- the completion
  callback fired right after drying started, arming smart-plug auto-off.

  Root cause: the tray-bearing branch of the AMS partial-update merge
  rebuilt the unit as {**ams_unit, "tray": merged_trays}, never spreading
  existing_unit. Tray-bearing partials carry no drying fields, so dry_time
  (and info) was dropped; the falling-edge detector read the absent field
  as 0 and saw a false 60->0 edge.

  - Merge: tray branch now spreads existing_unit first, preserving
    dry_time / info / humidity / temp across tray-only partials. Matches
    the no-tray branch. Also fixes dry_status/dry_sub_status UI flapping.
  - Detector: only evaluate the falling edge when dry_time is explicitly
    present and parseable; skip otherwise without updating state.

backend/tests/unit/services/test_bambu_mqtt.py

@@ -5132,3 +5132,23 @@ class TestDryingCompleteCallback:
         # And finishes.
         mqtt_client._handle_ams_data({"ams": [{"id": "0", "dry_time": 0, "tray": []}]})
         assert mqtt_client._drying_events == [0, 0]
+
+    def test_tray_only_partial_does_not_fake_completion(self, mqtt_client):
+        """#1462 — a tray-bearing partial update that omits dry_time must not
+        be read as dry_time=0. The pre-fix merge dropped dry_time on such
+        partials, so the falling-edge detector saw a 60→0 edge and fired a
+        false 'drying complete' seconds after drying started — which armed
+        smart-plug auto-off and killed the printer mid-cycle."""
+        # Drying active, 60 minutes remaining.
+        mqtt_client._handle_ams_data({"ams": [{"id": "0", "dry_time": 60, "tray": []}]})
+        assert mqtt_client._drying_events == []
+
+        # Printer sends a tray-bearing partial carrying NO dry_time field.
+        mqtt_client._handle_ams_data({"ams": [{"id": "0", "tray": []}]})
+        assert mqtt_client._drying_events == []
+        # dry_time survived the partial in the merged AMS state.
+        assert mqtt_client.state.raw_data["ams"][0]["dry_time"] == 60
+
+        # Drying genuinely finishes → the real edge still fires exactly once.
+        mqtt_client._handle_ams_data({"ams": [{"id": "0", "dry_time": 0, "tray": []}]})
+        assert mqtt_client._drying_events == [0]