Add standard names to romsds so decode_vertical_coords works as documented

[gaoflow]

Fixes #645.

Problem

decode_vertical_coords raises on the bundled romsds dataset, including in the documented example:

from cf_xarray.datasets import romsds
romsds.cf.decode_vertical_coords(outnames={"s_rho": "z_rho"})
# ValueError: The standard name for the 'depth' variable is not available.

romsds.s_rho declares formula_terms = "... eta: zeta depth: h ...", but the bundled h (depth) and zeta (eta) coordinates carry no standard_name. _derive_ocean_stdname needs those to look up the computed standard name, so it fails. Tellingly, the existing test_decode_vertical_coords worked around this by assigning the two standard_names inline before calling decode_vertical_coords — i.e. the bundled dataset was incomplete.

Fix

Give the bundled romsds h and zeta the standard names they need (sea_floor_depth_below_geopotential_datum and sea_surface_height_above_geopotential_datum — the same values the test was setting inline), so the dataset works as documented out of the box.

The now-redundant inline assignments are dropped from test_decode_vertical_coords, so the test exercises the bundled dataset directly and will catch a regression of this kind. decode_vertical_coords now yields z_rho with the computed standard name height_above_geopotential_datum.

Testing

test_parametric.py and test_accessor.py pass (205 passed, 24 skipped). The decode test fails without this change (the original ValueError) and passes with it. ruff check and ruff format are clean.

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This change was prepared by an AI agent under my direction; I reviewed and verified it.