Migrate users to tag-based structured_headers::Item constructors

Use tag-based constructors (`Item::string` and `Item::token`) introduced in cl/978581202.

This contains no intentional observable behavioral changes.

Tested:
    TAP --sample ran all affected tests and none failed
    http://test/OCL:979214341:BASE:979159221:1789055904004:a0c33791
PiperOrigin-RevId: 979320705

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 9427b74..a0aae85 100644
--- a/quiche/quic/tools/connect_udp_tunnel.cc
+++ b/quiche/quic/tools/connect_udp_tunnel.cc
@@ -383,11 +383,14 @@
   quiche::HttpHeaderBlock headers;
   headers[":status"] = status;
 
-  structured_headers::Item proxy_status_item(server_label_);
+  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(
-      std::string{proxy_status_error});
+      structured_headers::Item::string, proxy_status_error);
   structured_headers::Item proxy_status_details_item(
-      std::string{error_details});
+      structured_headers::Item::string, error_details);
   structured_headers::ParameterizedMember proxy_status_member(
       std::move(proxy_status_item),
       {{"error", std::move(proxy_status_error_item)},