Sensor init fails if unit of measurement is a string

[tr4nt0r]

When providing a unit in the tuya V2 quirk builder that is a string (instead of an Enum) the sensor init fails.
I used the unit lx, which is defined in the const LIGHT_LUX

    .tuya_sensor(
        dp_id=101,
        attribute_name="measured_value",
        type=t.uint16_t,
        fallback_name="Illuminance",
        device_class=SensorDeviceClass.ILLUMINANCE,
        state_class=SensorStateClass.MEASUREMENT,
        unit=LIGHT_LUX,
    )

Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/zha/application/gateway.py", line 335, in create_platform_entities
    platform_entity = platform_entity_class.create_platform_entity(
        *args, **kw_args
    )
  File "/usr/local/lib/python3.13/site-packages/zha/application/platforms/sensor/__init__.py", line 186, in create_platform_entity
    return cls(unique_id, cluster_handlers, endpoint, device, **kwargs)
  File "/usr/local/lib/python3.13/site-packages/zha/application/platforms/sensor/__init__.py", line 198, in __init__
    super().__init__(unique_id, cluster_handlers, endpoint, device, **kwargs)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/zha/application/platforms/__init__.py", line 297, in __init__
    self._init_from_quirks_metadata(entity_metadata)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/zha/application/platforms/sensor/__init__.py", line 220, in _init_from_quirks_metadata
    self._attr_native_unit_of_measurement = validate_unit(
                                            ~~~~~~~~~~~~~^
        entity_metadata.unit
        ^^^^^^^^^^^^^^^^^^^^
    ).value
    ^
  File "/usr/local/lib/python3.13/site-packages/zha/units.py", line 181, in validate_unit
    return UNITS_OF_MEASURE[type(external_unit).__name__](external_unit.value)
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'str'