Updates the keys the body names, merged over the stored field: a key left out keeps its value, and fieldOptions: null clears the options. idx may be a digit string (the old dashboard's inline cell sends one). standardFieldName is only taken when the field has none yet — a stored one is never overwritten. Responds with the field as stored. 404 product_category_field_not_found for an unknown id (legacy: not_found); 409 duplicated_field_option when the merged options repeat. Answers { data, metadata }; legacy answered the bare record.