Make constant fields available

We need access to the constant fields such as land_mask or terrain height. See, e.g., here https://github.com/MeteoSwiss/HiRAD-Gen/pull/19/files#r2257664640

Start by reviewing the linked pull request discussion and its changed files to understand how constant fields such as land_mask and terrain height are currently handled. Trace the relevant data-loading or model-input entry point, then verify that these fields can be accessed and are covered by an appropriate test or usage check.

Written by the indexing model from the issue text.