修复购物车应用 coupon 后 couponCode 为空

从 cart.coupon_code 读取优惠券码，而不是从不存在的 additional 字段取值。

Co-authored-by: Cursor <cursoragent@cursor.com>

packages/Webkul/BagistoApi/src/Dto/CartData.php

@@ -316,9 +316,7 @@ class CartData
         $data->baseGrandTotal = (float) ($cart->base_grand_total ?? 0);
         $data->formattedGrandTotal = core()->currency($cart->base_grand_total ?? 0);
 
-        $additional = $cart->additional ?
-            (is_string($cart->additional) ? json_decode($cart->additional, true) : $cart->additional) : [];
-        $data->couponCode = $additional['coupon_code'] ?? null;
+        $data->couponCode = $cart->coupon_code;
 
         // Gift card
         $data->giftcardNumber = $cart->giftcard_number ?? null;