fix(gost): mark node failed when transport detects relay error

When using relay connector with noDelay=false (default), connection
errors to the final target are deferred until first read/write during
Transport(). Previously the Transport() return value was ignored,
causing the marker to never be called for unreachable targets.

Now we capture the Transport() error and mark the node as failed,
enabling failover for subsequent connections.

go-gost/x/handler/forward/local/handler.go

@@ -252,7 +252,14 @@ func (h *forwardHandler) Handle(ctx context.Context, conn net.Conn, opts ...hand
 		}
 		defer cc.Close()
 
-		xnet.Transport(conn, cc)
+		if err := xnet.Transport(conn, cc); err != nil {
+			if marker := target.Marker(); marker != nil {
+				marker.Mark()
+				h.options.Logger.Debugf("[handler.transport] transport failed, marked node=%s count=%d err=%v",
+					target.Addr, marker.Count(), err)
+			}
+			return err
+		}
 		return nil
 	}