nit backend tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/users.rs

@@ -259,7 +259,7 @@ async fn test_user_endpoints(db: Pool<Postgres>) -> anyhow::Result<()> {
         let resp = authed(client().post(format!(
             "{global_base}/rename/newglobaluser@windmill.dev"
         )))
-        .json(&json!({"new_username": "renamed-user"}))
+        .json(&json!({"new_username": "renamed_user"}))
         .send()
         .await
         .unwrap();