fix: decode observations that omit solar_radiation (KeyError)

[brentb2529]

Problem

KeyError: 'solar_radiation' — Observation decode fails for stations that don't report solar radiation.

Some WeatherFlow/Tempest stations never report solar_radiation: it's absent from both obs[0] and outdoor_keys. Observation declares solar_radiation (along with brightness, dew_point, heat_index, pressure_trend, relative_humidity, uv) as a required field with no default. dataclasses_json decodes via a bare kvs[field.name] lookup, so a missing key raises KeyError and the entire observation is discarded. Downstream consumers (HA custom_components/tempest, core weatherflow_cloud) then go stale / unavailable.

This is the same bug class previously fixed for air_temperature (#128) and reported for brightness (home-assistant/core#156969) and pressure_trend.

Observed in HA as:

weatherflow4py.rest: Unable to convert data || {...} || to ||
  <class 'weatherflow4py.models.rest.observation.ObservationStationREST'> -- 'solar_radiation'

Fix

Make the sensor-derived observation fields optional with a None default, so a missing key decodes to None instead of raising. With dataclasses_json it's the default value (not just the | None hint) that makes the decoder skip the dict lookup.

• brightness, dew_point, heat_index, pressure_trend, relative_humidity, solar_radiation, uv → ... | None = None
• timestamp remains the only required field (present in every observation) and is moved first to satisfy dataclass field-ordering (a no-default field can't follow a defaulted one).
• uv_index_color / uv_index_exposure now guard against a None uv and return None.

No version bump — left to maintainer per the existing "bump version" commit convention.

Tests

• New regression test test_convert_json_to_observation_no_solar_radiation + fixture station_observation_no_solar_radiation.json (verbatim real payload from station 113968, which omits solar_radiation from both obs[0] and outdoor_keys).
• Full suite green locally: 98 passed, ruff check clean.

Closes the solar_radiation case of the recurring missing-field decode bug.

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