Fix retain time coordinate

[Alfie5640]

Description

Updated default profiled to enable add_valid_time_coord
When add_valid_time_coord=True, converting GRIB data containing a single valid time could fail during xarray conversion.

The failure occurred because scalar "valid_time" values were incorrectly assigned a coordinate dimension: ValueError: dimensions ('valid_time',) must have the same length as the number of data dimensions, ndim=0

This change updates coordinate dimension inference so scalar coordinate values are represented with no dimensions ( dims=() ), while non-scalar coordinates retain their inferred dimensions

Tests

Added a regression test covering the single valid time case to make sure that

• valid_time is retained as an auxiliary coordinate
• valid_time is not incorrectly promoted to a dimension
• scalar coordinate dimensions are represented correctly

Ran all tests with pytest

Contributor Declaration

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• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.