fix(inventory): emulate state=9 for bare-tray empty-slot signal on P1S/A1 (#1322)

  Follow-up to the #1322 root fix. Reporter @RosdasHH traced the raw MQTT
  payload and found that P1S and A1 Mini send only {"id": N} for a
  physically empty slot — no state, no tray_type, no other fields. Without
  that signal, the assign-spool path was firing one wasted MQTT publish per
  click on a truly-empty slot (firmware dropped it silently, but still).

  The AMS parser in printer_manager.py now detects the bare-tray shape and
  promotes it to state=9 — the firmware's explicit "no spool" code — which
  lets the existing state in {9, 10} short-circuit in the inventory route
  apply automatically.

  The detection is intentionally narrow:

      len(tray) == 1 and "id" in tray and state is None

  so the post-Reset-Slot A1 Mini BMCU case (populated payload with state=3
  and tray_type="") has more than one key and stays unaffected — the #1322
  root fix is preserved.

backend/app/services/printer_manager.py

@@ -768,6 +768,19 @@ def printer_state_to_dict(state: PrinterState, printer_id: int | None = None, mo
                 if k_value is None and cali_idx is not None and cali_idx in kprofile_map:
                     k_value = kprofile_map[cali_idx]
 
+                # P1S / A1 Mini physically-empty-slot signal (#1322 follow-up by
+                # @RosdasHH): for a truly empty slot the firmware sends only
+                # {"id": N} — no state, no tray_type, no anything else. Treat
+                # that as the firmware's "no spool" indicator (state=9) so the
+                # assign-spool path in inventory.py can short-circuit a MQTT
+                # publish the firmware would silently drop anyway. The
+                # post-"Reset Slot" A1 Mini BMCU case sends a populated payload
+                # (state=3, tray_type="") — different shape, doesn't match this
+                # guard, still attempts the MQTT push per the #1322 fix.
+                state_val = tray.get("state")
+                if state_val is None and len(tray) == 1 and "id" in tray:
+                    state_val = 9
+
                 trays.append(
                     {
                         "id": int(tray.get("id", 0)),
@@ -785,7 +798,7 @@ def printer_state_to_dict(state: PrinterState, printer_id: int | None = None, mo
                         "nozzle_temp_max": tray.get("nozzle_temp_max"),
                         "drying_temp": tray.get("drying_temp"),
                         "drying_time": tray.get("drying_time"),
-                        "state": tray.get("state"),
+                        "state": state_val,
                     }
                 )
             # Prefer humidity_raw (actual percentage) over humidity (index 1-5)

backend/tests/unit/services/test_printer_manager.py

@@ -741,6 +741,60 @@ class TestPrinterStateToDict:
         assert result["ams"][0]["tray"][0]["tag_uid"] is None
         assert result["ams"][0]["tray"][0]["tray_uuid"] is None
 
+    def test_bare_tray_emulates_state_9(self, mock_state):
+        """P1S / A1 Mini physically-empty-slot signal (#1322 follow-up by @RosdasHH):
+        the firmware sends only `{"id": N}` for a truly empty slot. Treat that as
+        the firmware's "no spool" state (state=9) so the inventory assign-spool
+        path can short-circuit the doomed MQTT publish.
+        """
+        mock_state.raw_data = {
+            "ams": [
+                {
+                    "id": 0,
+                    "tray": [
+                        {"id": 0, "state": 11, "tray_type": "PLA"},  # loaded slot
+                        {"id": 1},  # P1S empty-slot signal — only id
+                    ],
+                }
+            ]
+        }
+
+        result = printer_state_to_dict(mock_state)
+        trays = result["ams"][0]["tray"]
+
+        assert trays[0]["state"] == 11, "loaded slot keeps its firmware state"
+        assert trays[1]["state"] == 9, "bare {id} tray must be promoted to state=9"
+
+    def test_populated_payload_with_empty_state_3_is_not_promoted(self, mock_state):
+        """A1 Mini BMCU / P1S Standard AMS post-Reset-Slot case (#1322 root):
+        firmware sends state=3 + tray_type="" but with the FULL field set
+        populated. Must NOT be confused with the bare-tray empty signal —
+        else inventory.py would short-circuit MQTT and we'd reintroduce the
+        deadlock the #1322 fix removed.
+        """
+        mock_state.raw_data = {
+            "ams": [
+                {
+                    "id": 0,
+                    "tray": [
+                        {
+                            "id": 0,
+                            "state": 3,
+                            "tray_type": "",  # cleared
+                            "tray_color": "",
+                            "tag_uid": "0000000000000000",
+                            "remain": 0,
+                        }
+                    ],
+                }
+            ]
+        }
+
+        result = printer_state_to_dict(mock_state)
+        # state stays at 3 — the bare-tray promotion requires the dict to have
+        # ONLY the id key, not just empty/falsy values for the other fields.
+        assert result["ams"][0]["tray"][0]["state"] == 3
+
     def test_zero_tag_uid_becomes_none(self, mock_state):
         """Verify zero tag_uid is converted to None."""
         mock_state.raw_data = {