Use token for Proxy-Status error parameter in ConnectUdpTunnel.

[RFC 9209 § 2.1.1](https://www.rfc-editor.org/rfc/rfc9209.html#section-2.1.1)
specifies that the `error` parameter of the `Proxy-Status` HTTP header field
must be an sf-token rather than an sf-string.

PiperOrigin-RevId: 979795974

quiche/quic/tools/connect_udp_tunnel.cc

diff --git a/quiche/quic/tools/connect_udp_tunnel.cc b/quiche/quic/tools/connect_udp_tunnel.cc
index a0aae85..a72576d 100644
--- a/quiche/quic/tools/connect_udp_tunnel.cc
+++ b/quiche/quic/tools/connect_udp_tunnel.cc
@@ -385,10 +385,8 @@
 
   structured_headers::Item proxy_status_item(structured_headers::Item::string,
                                              server_label_);
-  // TODO(apaseltiner): RFC 9209, Section 2.1.1 specifies that `error` should be
-  // a token, not a string.
   structured_headers::Item proxy_status_error_item(
-      structured_headers::Item::string, proxy_status_error);
+      structured_headers::Item::token, proxy_status_error);
   structured_headers::Item proxy_status_details_item(
       structured_headers::Item::string, error_details);
   structured_headers::ParameterizedMember proxy_status_member(