How do I obtain the header value of SignedCookieJar?

[PuercoPop]

Summary

While writing tests against endpoints that need authentication I need to add a cookie header with the session id. Is there a way to obtain the header value from a cookie jar?

let jar = SignedCookieCookieJar::new();
// let session_id = "...";
// jar.add(Cookie::new("sid", session_id));
// ...    
let req = Request::builder()
    .method("GET")
    .uri("/admin")
    .header("Cookie", jar...)
    .unwrap();
let res = app.oneshot(req).await.unwrap();

axum version

axum: 0.8.1, axum-extra: 0.10.0