Add mg/m³ as a valid UOM for sensor/number Carbon Monoxide device class (#152456)

homeassistant/util/unit_conversion.py

@@ -168,6 +168,20 @@ class BaseUnitConverter:
         return (from_unit in cls._UNIT_INVERSES) != (to_unit in cls._UNIT_INVERSES)
 
 
+class CarbonMonoxideConcentrationConverter(BaseUnitConverter):
+    """Convert carbon monoxide ratio to mass per volume."""
+
+    UNIT_CLASS = "carbon_monoxide"
+    _UNIT_CONVERSION: dict[str | None, float] = {
+        CONCENTRATION_PARTS_PER_MILLION: 1,
+        CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER: 1.145609,
+    }
+    VALID_UNITS = {
+        CONCENTRATION_PARTS_PER_MILLION,
+        CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER,
+    }
+
+
 class DataRateConverter(BaseUnitConverter):
     """Utility to convert data rate values."""