main: 增强会话功能，支持消息管理和接口文档

- 添加 `last_message_id` 字段至 `chat_sessions` 表，更新其关联索引
- 实现会话更新接口，支持修改名称与状态并添加验证逻辑
- 增加会话列表接口，支持状态过滤与关键字查询
- 提供会话和消息相关的资源类和请求验证类
- 扩展 `ChatService` 服务层逻辑以处理会话更新和消息附加
- 编写测试用例以验证新功能的正确性
- 增加接口文档及 OpenAPI 规范文件，覆盖新增功能
- 更新数据库播种器，添加默认用户

app/Http/Resources/ChatSessionResource.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Http\Resources;
+
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+use Illuminate\Support\Str;
+
+/** @mixin \App\Models\ChatSession */
+class ChatSessionResource extends JsonResource
+{
+    /**
+     * @return array<string, mixed>
+     */
+    public function toArray(Request $request): array
+    {
+        $preview = $this->last_message_content ?? '';
+
+        return [
+            'session_id' => $this->session_id,
+            'session_name' => $this->session_name,
+            'status' => $this->status,
+            'last_seq' => $this->last_seq,
+            'created_at' => $this->created_at,
+            'updated_at' => $this->updated_at,
+            'last_message_at' => $this->last_message_at,
+            'last_message_preview' => $preview ? Str::limit($preview, 120) : '',
+            'last_message_role' => $this->last_message_role,
+            'last_message_type' => $this->last_message_type,
+        ];
+    }
+}