Derive the solar radiation device class from the resolved output unit

[dimatx]

DATA_POINT_SOLARRADIATION hardcodes device_class=irradiance, but its unit is
user-selectable via --output-unit-illuminance. Home Assistant accepts only W/m² and
BTU/(h⋅ft²) for irradiance, and only lx for illuminance, so every non-default
unit produces a pair HA rejects:

--output-unit-illuminance	device_class emitted	valid?
W/m² (default)	irradiance	✅
lx / klx / fc / kfc	irradiance	❌

MQTT discovery treats this as a schema error rather than a warning, so the sensor is
never created:

Error 'The unit of measurement `lx` is not valid together with device class `irradiance`'
when processing MQTT discovery message topic: 'homeassistant/sensor/<id>/solarradiation/config'

If the sensor already exists on the default unit, HA rejects the update instead and
keeps the old config, so the entity carries on reporting W/m² while lux values arrive
underneath.

Fix

Override the device class from the resolved unit, in the same style as
STATE_CLASS_OVERRIDES just below it:

UNIT_DEVICE_CLASS_OVERRIDES: dict[str, dict[str | None, str | None]] = {
    DATA_POINT_SOLARRADIATION: {
        UnitOfIlluminance.LUX: DeviceClass.ILLUMINANCE,
        UnitOfIlluminance.WATTS_PER_SQUARE_METER: DeviceClass.IRRADIANCE,
        UnitOfIlluminance.KILOLUX: None,
        UnitOfIlluminance.FOOT_CANDLES: None,
        UnitOfIlluminance.KILOFOOT_CANDLES: None,
    },
}

discovery.device_class = UNIT_DEVICE_CLASS_OVERRIDES.get(data_point_key, {}).get(
    discovery.unit_of_measurement, description.device_class
)

klx/fc/kfc map to None because HA has no device class that accepts them; omitting
device_class is valid and the sensor is created normally. Data points with no entry
keep their static device class, so the default output is unchanged.

Tests cover each unit, and the README's Illuminance section notes the resulting device
class. Verified against Home Assistant 2026.9.2.

Previously reported in #1297.

[bachya]

Thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (76451f9) to head (8b3715b).

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #1434   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           33        33           
  Lines         1797      1798    +1     
=========================================
+ Hits          1797      1798    +1     

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