Use public SolarPosition API for UWISC solar-angle derivation

[Copilot]

This PR addresses the review concern that preprocessing relied on private rex internals (SolarPosition._zenith / _azimuth), which are unstable across upstream upgrades. The implementation now uses the public SolarPosition interface while preserving existing 2D output behavior.

• Solar angle derivation now uses public API

  • Replaced private helper calls in BaseUwiscDataModel with:
    • SolarPosition(time_index, lat_lon_pairs).zenith
    • SolarPosition(time_index, lat_lon_pairs).azimuth
  • Lat/lon grids are flattened into (N, 2) pairs for SolarPosition, then reshaped back to the original spatial grid.

• Focused regression coverage

  • Added a preprocessing unit test that monkeypatches SolarPosition and verifies:
    • the expected (lat, lon) pair ordering passed into the constructor,
    • zenith and azimuth outputs are reshaped back to (south_north, west_east).

solar_pos = SolarPosition(
    self.time_index,
    np.column_stack((lats.ravel(), lons.ravel())),
)
zenith = np.asarray(solar_pos.zenith).reshape(lats.shape)
azimuth = np.asarray(solar_pos.azimuth).reshape(lats.shape)