chore(cart): remove redundant gift card code deduplication

Removes client-side duplicate filtering from `cartGiftCardCodesUpdate`.

## What
- Remove `uniqueCodes` filtering logic from `cartGiftCardCodeUpdateDefault.ts`

## Why
E2E testing confirmed the Storefront API handles duplicate gift card codes
gracefully (idempotent behavior). The client-side filtering was redundant
overhead that doesn't align with Hydrogen's thin wrapper pattern.

This has no observable effect on consumers - the API deduplicates identically.

Author: kdaviduik

packages/hydrogen/src/cart/queries/cartGiftCardCodeUpdateDefault.ts

@@ -16,22 +16,30 @@ export type CartGiftCardCodesUpdateFunction = (
   optionalParams?: CartOptionalInput,
 ) => Promise<CartQueryDataReturn>;
 
+/**
+ * Updates (replaces) gift card codes in the cart.
+ *
+ * This function no longer filters duplicate codes internally.
+ * To add codes without replacing, use `cartGiftCardCodesAdd` (API 2025-10+).
+ *
+ * @param {CartQueryOptions} options - Cart query options including storefront client and cart fragment.
+ * @returns {CartGiftCardCodesUpdateFunction} - Function accepting gift card codes array and optional parameters.
+ *
+ * @example Replace all gift card codes
+ * const updateGiftCardCodes = cartGiftCardCodesUpdateDefault({ storefront, getCartId });
+ * await updateGiftCardCodes(['SUMMER2025', 'WELCOME10']);
+ */
 export function cartGiftCardCodesUpdateDefault(
   options: CartQueryOptions,
 ): CartGiftCardCodesUpdateFunction {
   return async (giftCardCodes, optionalParams) => {
-    // Ensure the gift card codes are unique
-    const uniqueCodes = giftCardCodes.filter((value, index, array) => {
-      return array.indexOf(value) === index;
-    });
-
     const {cartGiftCardCodesUpdate, errors} = await options.storefront.mutate<{
       cartGiftCardCodesUpdate: CartQueryData;
       errors: StorefrontApiErrors;
     }>(CART_GIFT_CARD_CODE_UPDATE_MUTATION(options.cartFragment), {
       variables: {
         cartId: options.getCartId(),
-        giftCardCodes: uniqueCodes,
+        giftCardCodes,
         ...optionalParams,
       },
     });

packages/hydrogen/src/cart/queries/cartGiftCardCodesUpdateDefault.test.ts

@@ -27,4 +27,29 @@ describe('cartGiftCardCodesUpdateDefault', () => {
     expect(result.cart).toHaveProperty('id', CART_ID);
     expect(result.userErrors?.[0]).toContain(cartFragment);
   });
+
+  describe('no duplicate filtering (API 2025-10+)', () => {
+    it('should pass duplicate codes directly to API without filtering', async () => {
+      const updateGiftCardCodes = cartGiftCardCodesUpdateDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+      });
+
+      const codesWithDuplicates = ['GIFT123', 'GIFT123', 'WELCOME10'];
+      const result = await updateGiftCardCodes(codesWithDuplicates);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+    });
+
+    it('should delegate duplicate handling to API (case-insensitive normalization)', async () => {
+      const updateGiftCardCodes = cartGiftCardCodesUpdateDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+      });
+
+      const result = await updateGiftCardCodes(['gift123', 'GIFT123']);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+    });
+  });
 });