main: 增强会话功能，支持归档与消息检索

- 添加会话归档接口及相关服务逻辑，并确保幂等性
- 实现单条消息获取接口，校验消息所属会话
- 增加 SSE 增量推送与实时消息订阅功能
- 提供相关的测试用例覆盖新功能
- 更新接口文档，完善 OpenAPI 规范，新增多项示例

app/Http/Controllers/ChatSessionController.php

@@ -70,6 +70,17 @@ class ChatSessionController extends Controller
         return MessageResource::collection($messages)->response();
     }
 
+    public function showMessage(string $sessionId, string $messageId): JsonResponse
+    {
+        $message = $this->service->getMessage($sessionId, $messageId);
+
+        if (! $message) {
+            abort(404);
+        }
+
+        return (new MessageResource($message))->response();
+    }
+
     /**
      * 获取会话列表。
      *
@@ -109,4 +120,18 @@ class ChatSessionController extends Controller
 
         return (new ChatSessionResource($session))->response();
     }
+
+    public function show(string $sessionId): JsonResponse
+    {
+        $session = $this->service->getSessionWithLastMessage($sessionId);
+
+        return (new ChatSessionResource($session))->response();
+    }
+
+    public function archive(string $sessionId): JsonResponse
+    {
+        $session = $this->service->archiveSession($sessionId);
+
+        return (new ChatSessionResource($session))->response();
+    }
 }