refactor(checkout): simplify direct item handling in checkout process

src/app/core/services/checkout.service.ts

@@ -17,36 +17,13 @@ export interface DirectCheckoutItem {
   cantidad: number;
 }
 
-export interface UnavailableCheckoutItem {
-  index: number;
-  catalog_item_id: number;
-  variant_id: number | null;
-  requested_quantity: number;
-  available_quantity: number;
-  message: string;
-}
-
-export interface InsufficientStockResponse {
-  code: 'purchase.insufficient_stock';
-  message: string;
-  errors: Record<string, string[]>;
-  unavailable_items: UnavailableCheckoutItem[];
-}
-
-export function isInsufficientStockResponse(value: unknown): value is InsufficientStockResponse {
-  if (typeof value !== 'object' || value === null) return false;
-
-  const response = value as Partial<InsufficientStockResponse>;
-
-  return (
-    response.code === 'purchase.insufficient_stock' && Array.isArray(response.unavailable_items)
-  );
-}
-
 export type StartCheckoutPayload =
   | {
       cart_id: number;
     }
+  | {
+      direct_item: DirectCheckoutItem;
+    }
   | {
       direct_items: DirectCheckoutItem[];
     };