fix: add state_class for co2-density sensor (#1631)

Add co2-density property mapping to SPEC_PROP_TRANS_MAP to enable
statistics recording for CO2 sensors.

Changes:
- Import CONCENTRATION_PARTS_PER_MILLION from homeassistant.const
- Add co2-density mapping with:
  - device_class: SensorDeviceClass.CO2
  - state_class: SensorStateClass.MEASUREMENT
  - unit_of_measurement: CONCENTRATION_PARTS_PER_MILLION (ppm)

This ensures CO2 sensors (like those from CGDN1 device) have proper
device classification and Home Assistant can record long-term statistics.

Fixes #1630

custom_components/xiaomi_home/miot/specs/specv2entity.py

@@ -51,6 +51,7 @@ from homeassistant.components.event import EventDeviceClass
 from homeassistant.components.binary_sensor import BinarySensorDeviceClass
 
 from homeassistant.const import (CONCENTRATION_MICROGRAMS_PER_CUBIC_METER,
+                                 CONCENTRATION_PARTS_PER_MILLION,
                                  EntityCategory, LIGHT_LUX, UnitOfEnergy,
                                  UnitOfPower, UnitOfElectricCurrent,
                                  UnitOfElectricPotential, UnitOfTemperature,
@@ -642,6 +643,12 @@ SPEC_PROP_TRANS_MAP: dict = {
             'entity': 'sensor',
             'state_class': SensorStateClass.MEASUREMENT
         },
+        'co2-density': {
+            'device_class': SensorDeviceClass.CO2,
+            'entity': 'sensor',
+            'state_class': SensorStateClass.MEASUREMENT,
+            'unit_of_measurement': CONCENTRATION_PARTS_PER_MILLION
+        },
         'battery-level': {
             'device_class': SensorDeviceClass.BATTERY,
             'entity': 'sensor',