fix(gateway): use exact payload size filter and improve robustness

- Use exact 12-byte payload size filter instead of minimum to avoid
  false positives from non-Thingy devices with same company_id
- Discard CO2 value 0xFFFFFFFF indicating sensor not ready or failed
- Downgrade unknown key and empty payload logs to DEBUG level
- Add on_connect callback to confirm successful broker connection
- Add on_publish callback to confirm message delivery to broker
- Rename MIN_PAYLOAD_SIZE to EXPECTED_PAYLOAD_SIZE for clarity

Validated: data received and confirmed by broker after Thingy reboot

gateway/gateway.py

@@ -24,6 +24,13 @@ class Gateway:
     KEY_TEMP     = 0x03
     KEY_CO2      = 0x04
 
+    # Sentinel value indicating sensor failure or not ready
+    INVALID_VALUE = 0xFFFFFFFF
+
+    # Expected payload size in bytes:
+    # 4 keys (1B each) + window(1B) + humidity(1B) + temp(2B) + co2(4B) = 12 bytes
+    EXPECTED_PAYLOAD_SIZE = 12
+
     def __init__(self, config: dict):
         self.gateway_id  = config["gateway_id"]
         self.mqtt_broker = config["mqtt"]["broker"]
@@ -81,6 +88,9 @@ class Gateway:
           0x02 : humidity (1 byte, integer %)
           0x03 : temperature (2 bytes big-endian, integer / 10 = degrees C)
           0x04 : CO2 ppm (4 bytes big-endian, integer)
+
+        Values equal to 0xFFFFFFFF indicate sensor failure or not ready
+        and are discarded.
         """
         result = {}
         i = 0  # no preamble to skip — company_id is not in raw bytes
@@ -100,10 +110,16 @@ class Gateway:
                 result["temp"] = raw / 10
                 i += 2
             elif key == self.KEY_CO2 and i + 3 < len(data):
-                result["co2_ppm"] = int.from_bytes(data[i:i+4], byteorder='big')
+                co2 = int.from_bytes(data[i:i+4], byteorder='big')
+                # 0xFFFFFFFF indicates sensor not ready or failed
+                if co2 != self.INVALID_VALUE:
+                    result["co2_ppm"] = co2
+                else:
+                    log.debug(f"CO2 sensor not ready — discarding value 0xFFFFFFFF")
                 i += 4
             else:
-                log.warning(f"Unknown key 0x{key:02x} at offset {i-1}")
+                # Unknown key — likely a non-Thingy device, ignore silently
+                log.debug(f"Unknown key 0x{key:02x} at offset {i-1}")
                 break
         return result
 
@@ -132,15 +148,19 @@ class Gateway:
         if 0xffff not in adv_data.manufacturer_data:
             return
 
-        # company_id 0xffff is defined in the firmware spec
-        # the raw bytes do not include the company_id itself
         raw = adv_data.manufacturer_data[0xffff]
 
+        # Filter on exact payload size to avoid false positives from
+        # other BLE devices using the same company_id
+        if len(raw) != self.EXPECTED_PAYLOAD_SIZE:
+            log.debug(f"{device.address} | ignored — unexpected payload size: {len(raw)}")
+            return
+
         log.debug(f"{device.address} | Thingy detected, raw: {list(raw)}")
 
         data = self.decode_payload(raw)
         if not data:
-            log.warning(f"{device.address} | empty decoded payload")
+            log.debug(f"{device.address} | empty decoded payload — ignored")
             return
 
         log.debug(f"{device.address} | decoded: {data}")