fix: restore leader change detection in KeepConnected stream loop

Critical fix: Leader change detection was accidentally removed from the streaming loop
- Master can announce leader changes during an active KeepConnected stream
- Without this check, client continues talking to non-leader until connection breaks
- This can lead to stale data or operational errors

The check needs to be in TWO places:
1. Initial response (lines 178-187): Detect redirect on first connect
2. Stream loop (lines 203-209): Detect leader changes during active stream

Restored the loop check that was accidentally removed during refactoring.
This ensures the client immediately reconnects to new leader when announced.

weed/wdclient/masterclient.go

@@ -199,6 +199,14 @@ func (mc *MasterClient) tryConnectToMaster(ctx context.Context, master pb.Server
 			// }
 
 			if resp.VolumeLocation != nil {
+				// Check for leader change during the stream
+				// If master announces a new leader, reconnect to it
+				if resp.VolumeLocation.Leader != "" && string(mc.GetMaster(ctx)) != resp.VolumeLocation.Leader {
+					glog.V(0).Infof("currentMaster %v redirected to leader %v", mc.GetMaster(ctx), resp.VolumeLocation.Leader)
+					nextHintedLeader = pb.ServerAddress(resp.VolumeLocation.Leader)
+					stats.MasterClientConnectCounter.WithLabelValues(stats.RedirectedToLeader).Inc()
+					return nil
+				}
 				mc.updateVidMap(resp)
 			}
 			if resp.ClusterNodeUpdate != nil {