refactor(api): 将WebChatUpgrade接口更新为Upgrade_V2并调整相关字段

更新API端点从WebChatUpgrade改为Upgrade_V2，同时将响应字段Data更名为Message
修改涉及README文档、API规范、控制器和客户端代码，保持接口变更的一致性

spec/rcu-upgrade-flow.yaml

@@ -5,7 +5,7 @@ info:
   description: Specification for RCU Upgrade Service API interactions and data models.
 
 paths:
-  /api/WebChatUpgrade:
+  /api/Upgrade_V2:
     post:
       summary: Trigger Upgrade
       operationId: triggerUpgrade
@@ -63,7 +63,7 @@ components:
       properties:
         IsSuccess:
           type: boolean
-        Data:
+        Message:
           type: string
       required:
         - IsSuccess