fix(tests): add user_agent field to crawler_browser_smoke LaunchOptions

Follow-up to the UA fix — the ignored browser smoke test builds LaunchOptions
as a struct literal and needs the new user_agent field to compile.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/crawler_browser_smoke.rs

@@ -113,6 +113,7 @@ async fn extra_args_reach_chromium() {
     let options = LaunchOptions {
         mode: browser::BrowserMode::Headless,
         extra_args: vec![format!("--user-agent={UA}")],
+        user_agent: None,
     };
     let handle = browser::launch(options).await.expect("launch with extra args");