feat: ClientApi.php Swagger 점검 및 개선 (Phase 3-3)

- ClientStoreRequest.php 생성 (검증 로직 분리)
- ClientUpdateRequest.php 생성 (검증 로직 분리)
- ClientController.php FormRequest 적용 및 패턴 통일
- lang/ko/message.php client 메시지 키 추가
- ApiResponse::handle 패턴 통일 (메시지 두 번째 인자)
- SAM API Development Rules 준수 완료

app/Http/Requests/Client/ClientStoreRequest.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Http\Requests\Client;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class ClientStoreRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'client_group_id' => 'nullable|integer',
+            'client_code' => 'required|string|max:50',
+            'name' => 'required|string|max:100',
+            'contact_person' => 'nullable|string|max:100',
+            'phone' => 'nullable|string|max:20',
+            'email' => 'nullable|email|max:100',
+            'address' => 'nullable|string|max:255',
+            'is_active' => 'nullable|in:Y,N',
+        ];
+    }
+}