Refactor selectlist attach feature

[LionelZoubritzky-IGN]

Fix two code inconsistencies:

• 77570a9 merges the two existing implementations of the select: one exists for the GpfGetFeatures codepath and another for GpfGetFeatureById, while they should be identical.
• d74b22b renames attachFeatureRefs into postProcessFeatureCollection to more accurately reflect that this function handles all the post-processing, and does not only attach feature refs anymore.

Pure refactors, no impact on behavior.

[esgn]

The default parameter geometryProperty = getGeometryProperty(featureType) is evaluated whenever the argument is not provided, even when the geometry is not needed (no select, no spatial_extras). Yet getGeometryProperty throws on a type without any geometry property.

Consequence: a bare gpf_get_feature_by_id on wfs_scot:doc_urba or wfs_scot:doc_urba_com (the 2 geometry-less types in the embedded catalog) now fails before even issuing the WFS request, whereas it worked on main. The old buildPropertyName in byId.ts only resolved the geometry when necessary — that was the invariant documented by the comment removed along with the function.

[LionelZoubritzky-IGN]

Thank you for catching that regression! Addressed and tested in 25871c3

Additionally, when doing a GetFeatureById with empty select and spatial_extra, the query used to have an empty propertyName, which implies fetching the entire geometry, and then discarding it (and replacing it will null). The refactor already changed that, and the latest commit tests this case. It also add tests for the case of collection without geometries, like wfs_scot:doc_urba.