test: fix flaky test_launch_options_from_env_defaults due to missing EnvGuard (#763)

The test was reading AGENT_BROWSER_HEADED without holding ENV_MUTEX,
causing a race with test_launch_options_from_env_headed_flag when
tests run in parallel.

cli/src/native/actions.rs

@@ -5267,6 +5267,7 @@ mod tests {
 
     #[test]
     fn test_launch_options_from_env_defaults() {
+        let _guard = EnvGuard::new(&["AGENT_BROWSER_HEADED"]);
         let opts = launch_options_from_env();
         assert!(opts.headless);
         assert!(opts.args.is_empty());