Sets the value of a single custom field on a position, identified by its custom field ID.

The submitted value is validated against the custom field's configured data_type: string/text/date require a string, dropdown/radio require a string matching one of the field's configured options.values, and currency requires a number; fields with data_type: attachment cannot be updated through this endpoint. On success, returns the full array of the position's custom fields (each with _id, name, type, data_type, value, requirement, and options), not just the one that was updated.