fix(server-party): session 事件订阅断开后自动重连

Message Router 重启后，server-party 的 gRPC stream 会断开，
之前的实现会直接退出 goroutine 导致无法收到新的 session 事件。

修改内容：
- 添加自动重连逻辑，stream 断开时会尝试重新订阅
- 使用指数退避策略，从 1 秒到最大 30 秒
- 重连成功后重置退避时间

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

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

backend/mpc-system/services/server-party/adapters/output/grpc/message_router_client.go

@@ -385,7 +385,7 @@ func (c *MessageRouterClient) UpdateNotificationChannels(
 	return c.RegisterPartyWithNotification(ctx, partyID, partyRole, version, notification)
 }
 
-// SubscribeSessionEvents subscribes to session lifecycle events
+// SubscribeSessionEvents subscribes to session lifecycle events with auto-reconnect
 func (c *MessageRouterClient) SubscribeSessionEvents(
 	ctx context.Context,
 	partyID string,
@@ -396,7 +396,7 @@ func (c *MessageRouterClient) SubscribeSessionEvents(
 		EventTypes: []string{}, // Subscribe to all event types
 	}
 
-	// Create a streaming connection
+	// Create initial streaming connection
 	stream, err := c.createSessionEventStream(ctx, req)
 	if err != nil {
 		logger.Error("Failed to subscribe to session events",
@@ -408,8 +408,12 @@ func (c *MessageRouterClient) SubscribeSessionEvents(
 	logger.Info("Subscribed to session events",
 		zap.String("party_id", partyID))
 
-	// Start goroutine to receive events
+	// Start goroutine to receive events with auto-reconnect
 	go func() {
+		currentStream := stream
+		reconnectBackoff := time.Second // Start with 1 second backoff
+		maxBackoff := 30 * time.Second
+
 		for {
 			select {
 			case <-ctx.Done():
@@ -418,27 +422,60 @@ func (c *MessageRouterClient) SubscribeSessionEvents(
 				return
 			default:
 				event := &router.SessionEvent{}
-				err := stream.RecvMsg(event)
+				err := currentStream.RecvMsg(event)
 				if err == io.EOF {
-					logger.Info("Session event stream ended",
+					logger.Warn("Session event stream ended, reconnecting...",
 						zap.String("party_id", partyID))
-					return
-				}
-				if err != nil {
-					logger.Error("Error receiving session event",
+				} else if err != nil {
+					logger.Warn("Error receiving session event, reconnecting...",
 						zap.Error(err),
 						zap.String("party_id", partyID))
-					return
+				} else {
+					// Successfully received event, reset backoff
+					reconnectBackoff = time.Second
+
+					logger.Info("Received session event",
+						zap.String("event_type", event.EventType),
+						zap.String("session_id", event.SessionId),
+						zap.String("party_id", partyID))
+
+					// Call event handler
+					if eventHandler != nil {
+						eventHandler(event)
+					}
+					continue
 				}
 
-				logger.Info("Received session event",
-					zap.String("event_type", event.EventType),
-					zap.String("session_id", event.SessionId),
-					zap.String("party_id", partyID))
+				// Reconnect with exponential backoff
+				for {
+					select {
+					case <-ctx.Done():
+						return
+					case <-time.After(reconnectBackoff):
+						logger.Info("Attempting to reconnect session event stream",
+							zap.String("party_id", partyID),
+							zap.Duration("backoff", reconnectBackoff))
 
-				// Call event handler
-				if eventHandler != nil {
-					eventHandler(event)
+						newStream, err := c.createSessionEventStream(ctx, req)
+						if err != nil {
+							logger.Error("Failed to reconnect session event stream",
+								zap.Error(err),
+								zap.String("party_id", partyID))
+							// Increase backoff for next attempt
+							reconnectBackoff = reconnectBackoff * 2
+							if reconnectBackoff > maxBackoff {
+								reconnectBackoff = maxBackoff
+							}
+							continue
+						}
+
+						logger.Info("Successfully reconnected to session events",
+							zap.String("party_id", partyID))
+						currentStream = newStream
+						reconnectBackoff = time.Second // Reset backoff on success
+						break
+					}
+					break
 				}
 			}
 		}