fix(mpc-service): 修复 WebSocket 导入方式

将 `import WebSocket from 'ws'` 改为 `import * as WebSocket from 'ws'`
以兼容 CommonJS 模块系统

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/services/mpc-service/src/infrastructure/external/mpc-system/message-router-client.ts

@@ -6,7 +6,7 @@
 
 import { Injectable, Logger, OnModuleInit, OnModuleDestroy } from '@nestjs/common';
 import { ConfigService } from '@nestjs/config';
-import WebSocket from 'ws';
+import * as WebSocket from 'ws';
 import { EventEmitter } from 'events';
 
 export interface MPCMessage {