fix(service-party-app): 修复 handleIncomingMessage 字段名 snake_case 问题

问题：
- gRPC proto-loader 使用 keepCase: true，返回 snake_case 字段名
- tss-handler.ts 的 handleIncomingMessage 期望 camelCase 字段名
- 导致 message_id, from_party, is_broadcast 等字段无法正确读取
- TSS 进程无法收到正确的消息，keygen 无法完成

修复：
- handleIncomingMessage 参数改为 snake_case (message_id, from_party, is_broadcast)
- 内部转换为 camelCase 格式后处理

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-app/electron/modules/tss-handler.ts

@@ -384,13 +384,23 @@ export class TSSHandler extends EventEmitter {
    * 1. isPrepared=true, isRunning=false: 预订阅阶段，缓冲消息
    * 2. isPrepared=true, isRunning=true, isProcessReady=false: 进程启动中，缓冲消息
    * 3. isPrepared=true, isRunning=true, isProcessReady=true: 进程就绪，直接发送
+   *
+   * Note: gRPC message uses snake_case field names due to keepCase: true in proto-loader
    */
-  private handleIncomingMessage(message: {
+  private handleIncomingMessage(rawMessage: {
-    messageId: string;
+    message_id: string;
-    fromParty: string;
+    from_party: string;
-    isBroadcast: boolean;
+    is_broadcast: boolean;
     payload: Buffer;
   }): void {
+    // 转换为内部使用的 camelCase 格式
+    const message = {
+      messageId: rawMessage.message_id,
+      fromParty: rawMessage.from_party,
+      isBroadcast: rawMessage.is_broadcast,
+      payload: rawMessage.payload,
+    };
+
     // 消息去重检查
     if (this.database && message.messageId) {
       if (this.database.isMessageProcessed(message.messageId)) {