fix(tuic): end a UDP response relay with its association

Dissociating a UDP association closed its SOCKS session, but the
response relay could be blocked opening a stream on credit the client
never grants, and only the connection's end released it. A client could
leave one such goroutine per Packet/Dissociate round, past the cap on
live associations. Each relay now runs under a context the association's
removal cancels, whichever path removes it.

internal/tuic/server.go

@@ -922,6 +922,11 @@ func (s *Server) relayUDPResponses(
 	sess *udpRelaySession,
 ) {
 	defer associations.release(assocID, association)
+	ctx, stop := context.WithCancel(ctx)
+	defer stop()
+	if !associations.attachResponseStop(assocID, association, stop) {
+		return
+	}
 	buf := make([]byte, s.maxUdpRelayPacketSize+263)
 	var nextPktID uint16
 	for {