fix(handler/serial,sni): 5 code-review fixes - xnet.Pipe errors, (nil,nil) warning, readTimeout comment, defer deadline, clean close

- serial/handler.go: capture xnet.Pipe return value in Handle() and forwardSerial()
- serial/handler.go: log Warnf when chain dial returns (nil,nil) before falling back
- sni/handler.go: use defer for read deadline reset to prevent stale deadline
- sni/handler.go: unrecognized protocol returns nil (clean close) instead of error
- sni/metadata.go: update readTimeout comment to reflect dual usage (deadline + SnifferBuilder)

handler/sni/handler.go

@@ -122,12 +122,10 @@ func (h *sniHandler) Handle(ctx context.Context, conn net.Conn, opts ...handler.
 
 	if h.md.readTimeout > 0 {
 		conn.SetReadDeadline(time.Now().Add(h.md.readTimeout))
+		defer conn.SetReadDeadline(time.Time{})
 	}
 	br := bufio.NewReader(conn)
 	proto, sniffErr := sniffing.Sniff(ctx, br)
-	if h.md.readTimeout > 0 {
-		conn.SetReadDeadline(time.Time{})
-	}
 	if sniffErr != nil {
 		log.Debugf("sniff: %v", sniffErr)
 	}
@@ -189,7 +187,8 @@ func (h *sniHandler) Handle(ctx context.Context, conn net.Conn, opts ...handler.
 			sniffing.WithLog(log),
 		)
 	default:
-		return errors.New("unknown traffic")
+			log.Debugf("unrecognized traffic from %s", conn.RemoteAddr())
+			return nil
 	}
 }