Function parse_keyset_sort 

pub fn parse_keyset_sort(
    sort: Option<&str>,
    primary_key: &str,
    nullable_of: impl Fn(&str) -> Option<bool>,
) -> Result<Vec<KeysetColumn>, CratestackError>

Resolve sort ("-createdAt,name"; None = unsorted) into keyset columns, appending primary_key ascending when absent so the order is total. nullable_of returns Some(is_nullable) for a sortable scalar column and None for anything else. Relation-path (a.b) terms, unknown or duplicate columns and empty terms are BadRequest.