Changes to List billing group credits

the credits/items/code response property's maxLength was unset from 1024 for the response status 200

removed the optional property errors from the response with the 200 status

removed the optional property message from the response with the 200 status

the response property credits/items/code became required for the status 200

the response property credits/items/expire_time became required for the status 200

the response property credits/items/remaining_value became required for the status 200

the response property credits/items/start_time became required for the status 200

the response property credits/items/type became required for the status 200

the response property credits/items/value became required for the status 200