CartSession headers issue with big pipe sessionless

Hello, I got same error. After the payment complete the session was closed and when I try to render order for search API, I got an error "headers already sent" On my side i have this error on method:

class CartSession
...
public function hasCartId($cart_id, $type = self::ACTIVE) {
    $key = $this->getSessionKey($type);
    $ids = $this->session->get($key, []);
    return in_array($cart_id, $ids);
  }

So, I have a little bit changed your fix.

Initially committed this without paying attention to the failing tests which I thought were the usual random test failures we're experiencing... Also, there's a small phpcs violation, so reverted this for now.

It seems like test failures are happening due to isStarted() returning FALSE if the session was started lazily.
The related core issue is #3044387: No way to tell if session has just started. → .

As strange as it may sound, we can't rely on isStarted() to check if a user session exists. That's due to the session lazy load logic implemented in Drupal core.

An alternative is to check for the session cookie's existence (depending on implementation, it could be something other than a cookie).
Said, if there's no cookie, we don't want to lazy start the session (and so we won't send a new session cookie to the browser which is exactly what this issue aims to fix).

Attached patches are:
- just the test changes alone (had to simulate cookie existence);
- test + issue fix.