XHTTP/3 client: Actively close underlying QUIC & UDP (#6332)

Fixes https://github.com/XTLS/Xray-core/issues/6328#issuecomment-4730379021

transport/internet/splithttp/client.go

@@ -9,6 +9,7 @@ import (
 	"net/http/httptrace"
 	"sync"
 
+	"github.com/apernet/quic-go/http3"
 	"github.com/xtls/xray-core/common"
 	"github.com/xtls/xray-core/common/buf"
 	"github.com/xtls/xray-core/common/errors"
@@ -176,6 +177,15 @@ func (c *DefaultDialerClient) PostPacket(ctx context.Context, url string, sessio
 	return nil
 }
 
+// HTTP/1.1 and HTTP/2 will close itself, we only handle HTTP/3 here
+func (c *DefaultDialerClient) Close() error {
+	transport := c.client.Transport
+	if h3Transport, ok := transport.(*http3.Transport); ok {
+		h3Transport.Close()
+	}
+	return nil
+}
+
 type WaitReadCloser struct {
 	Wait chan struct{}
 	io.ReadCloser