feat: Product API에 FormRequest 및 i18n 적용

- ProductStoreRequest, ProductUpdateRequest 생성
- ProductController에 FormRequest 적용
- 하드코딩된 메시지를 i18n 키로 변경
- lang/ko/message.php에 product 관련 메시지 키 추가
- SAM API Development Rules 준수

Phase 3-1: ProductApi.php Swagger 점검 완료

app/Http/Requests/Product/ProductUpdateRequest.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Http\Requests\Product;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class ProductUpdateRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'code' => 'sometimes|string|max:30',
+            'name' => 'sometimes|string|max:100',
+            'category_id' => 'sometimes|integer',
+            'product_type' => 'sometimes|string|max:30',
+            'attributes' => 'nullable|array',
+            'description' => 'nullable|string|max:255',
+            'is_sellable' => 'nullable|in:0,1',
+            'is_purchasable' => 'nullable|in:0,1',
+            'is_producible' => 'nullable|in:0,1',
+            'is_active' => 'nullable|in:0,1',
+        ];
+    }
+}