Fix ICON Reader

[rajeeja]

• _icon_to_ugrid_dims now derives the mapping from the actual ICON coords and connectivity (including nv→n_max_face_nodes, nc→two, etc.), and falls back to common names.
• With the full map, _map_dims_to_ugrid cleanly swaps all ICON dims to UGRID for both grid and data, so face coords are consistently normalized. When cross_section rewrites the lon/lat coord keys to the new steps dimension, there’s no leftover clon/clat tied to n_face, and validation passes.

This makes ICON I/O robust across variants and preventing future coord/dim mismatches.

Fixes #1425

@tdoerff please verify.

[erogluorhan]

Seems like we are still having the same issues with cross-sections on the dataset @tdoerff provided through this branch. I am debugging your changes.

[erogluorhan]

Looks great! I've re-run link-check that failed. Once it passes, we can merge this