Gets the options presented to the user, in Opt order (never sorted); empty when there is no Opt.
public IReadOnlyList<PdfChoiceOption> Options { get; }
ISO 32000-2 §12.7.5.4, Table 234: Opt is not inheritable, but an ancestor's is used when the field has none, as viewers
do, with a ChoiceOptionsInherited diagnostic. An element that is neither a text string nor a pair of them reads as an
empty option, so indices stay aligned, with a ChoiceOptionInvalid diagnostic.