feat(purchase): implement InsufficientStockException for handling stock errors in checkout process

app/Domains/Purchase/Exceptions/InsufficientStockException.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Domains\Purchase\Exceptions;
+
+use RuntimeException;
+
+class InsufficientStockException extends RuntimeException
+{
+    /**
+     * @param  array<int, array{
+     *     index: int,
+     *     catalog_item_id: int,
+     *     variant_id: int|null,
+     *     requested_quantity: int,
+     *     available_quantity: int,
+     *     message: string
+     * }>  $unavailableItems
+     */
+    public function __construct(
+        public readonly array $unavailableItems,
+    ) {
+        parent::__construct(
+            collect($unavailableItems)->pluck('message')->unique()->implode(' '),
+        );
+    }
+
+    /** @return array<string, array<int, string>> */
+    public function errors(): array
+    {
+        return collect($this->unavailableItems)
+            ->mapWithKeys(fn (array $item): array => [
+                "direct_items.{$item['index']}.cantidad" => [$item['message']],
+            ])
+            ->all();
+    }
+}