feat(stock): add handling for unavailable variants in product selection

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

@@ -17,6 +17,32 @@ 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;