Store the geometry label coordinates as Zarr string #356

Reported in #349 (Z1).

Three label coordinates under /conditions/geometry use data types that are not in the Zarr v3 core:

null_terminated_bytes is not in the zarr-extensions registry (see zarr-developers/zarr-extensions#60, open). A strict reader must refuse an unknown data type. These data types come from the native CPM Zarr v3 products (reported to CPM in https://gitlab.eopf.copernicus.eu/cpm/eopf-cpm/-/work_items/1268; in Zarr v2 products the labels are unicode). Our engine passes them through without conversion.

Proposal

Before the write, cast these coordinates to Python str (numpy object or StringDType), so that zarr-python writes the registered string data type. Decode bytes as ASCII.

Impact

• titiler-eopf reads the labels with v.decode() if bytes else str(v) (titiler/eopf/openeo/angles.py:366-374), so this works. Add a test with a variable-length string in tests/test_angles.py.
• rio-tiler shows b'b04' today. After the change it shows b04.
• Check that zarrita (OpenLayers) and GDAL read string.