Pin why the bracketed origin is rejected, not just that it is

Asserting only the failure and an empty sent buffer is a weak signal: a
stricter host validator (say, LDH-only) would reject these too, for the
wrong reason, and pass. Checking the message keeps the assertion tied to
the IPv6 case; a substring, so the wording stays free.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

src/htsselftest.c

@@ -1510,6 +1510,22 @@ static int st_socks5(httrackp *opt, int argc, char **argv) {
     assertf(io.sent_len == 0);
   }
 
+  /* the request is always ATYP=domain, which cannot carry an IPv6 literal: a
+     bracketed origin is rejected rather than sent as a bogus domain name. The
+     msg check pins the reason: a stricter host validator would also reject
+     these, but for the wrong cause. */
+  io.reply = script;
+  io.reply_len = len;
+  assertf(socks5_handshake_scripted(opt, "[::1]", proxy, &io) == 0);
+  assertf(io.sent_len == 0);
+  assertf(strstr(io.msg, "IPv6") != NULL);
+  io.reply = script;
+  io.reply_len = len;
+  assertf(socks5_handshake_scripted(opt, "[2001:db8::1]:8443", proxy, &io) ==
+          0);
+  assertf(io.sent_len == 0);
+  assertf(strstr(io.msg, "IPv6") != NULL);
+
   printf("socks5 self-test OK\n");
   return 0;
 }