The ctl roundtrip test sets relay_url

Config grew relay_url in 98db877; the test's literal never did, so CI
had not compiled since.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MznPTC31w5mSUK4XMehcpi

varde-ctl/tests/roundtrip.rs

@@ -18,6 +18,7 @@ fn ctl_status_round_trips() {
         max_download_bytes_per_sec: 0,
         discovery: false,
         wan_upload: false,
+        relay_url: None,
         gc_interval_secs: 300,
     };