Get Custom Field search options

Returns the values the search builder can suggest for this custom field, so exact-match operators can offer a select instead of a free-text box.

select_supported is false when the field isn’t backed by a usable option set (not a searchable string field with the select input type, beta flag off, never harvested, or over the distinct value limit) — the frontend then renders free text. When it is true, options may still be empty: the field is select-backed but no values exist yet.