fix(grpc-client): add connection check and better error handling in subscribeMessages

Additional safeguards to prevent "CANCELLED: Cancelled on client" error:

1. Add `this.connected` check at the start of subscribeMessages()
2. Set messageStream to null after canceling old stream
3. Wrap new stream creation in try-catch to handle creation errors
4. Add logging for ignored cancel errors

These changes ensure that:
- subscribeMessages won't proceed if connection is lost
- Old stream is fully cleaned up before creating new one
- Errors during stream creation are properly caught and logged

🤖 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/grpc-client.ts

@@ -550,6 +550,10 @@ export class GrpcClient extends EventEmitter {
       throw new Error('Not connected');
     }
 
+    if (!this.connected) {
+      throw new Error('gRPC client not connected');
+    }
+
     // 保存订阅状态（用于重连后恢复）
     this.activeMessageSubscription = { sessionId, partyId };
 
@@ -560,15 +564,22 @@ export class GrpcClient extends EventEmitter {
       try {
         oldStream.cancel();
       } catch (e) {
-        // 忽略
+        console.log('[gRPC] Ignored error while canceling old message stream:', (e as Error).message);
       }
+      this.messageStream = null;
     }
 
-    this.messageStream = (this.client as grpc.Client & { subscribeMessages: (req: unknown) => grpc.ClientReadableStream<MPCMessage> })
-      .subscribeMessages({
-        session_id: sessionId,
-        party_id: partyId,
-      });
+    try {
+      this.messageStream = (this.client as grpc.Client & { subscribeMessages: (req: unknown) => grpc.ClientReadableStream<MPCMessage> })
+        .subscribeMessages({
+          session_id: sessionId,
+          party_id: partyId,
+        });
+    } catch (e) {
+      console.error('[gRPC] Failed to create message stream:', (e as Error).message);
+      this.activeMessageSubscription = null;
+      throw e;
+    }
 
     // 保存当前流的引用，用于在事件处理器中检查是否是当前活跃的流
     const currentStream = this.messageStream;