Include session_cookie_token_prefix in KAS session cookie

What does this MR do and why?

Builds upon the changes added in !145178 (merged) to add the configured prefix to the _kas_session cookie used for the Kubernetes Dashboard. This will allow routing dashboard requests to the correct KAS/cell.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure GDK with KAS and an agent that is set up for user access
2. Add gdk.yml config to use a cookie prefix:

    gitlab:
      rails:
        session_store:
          session_cookie_token_prefix: 'test_prefix_'

3. Run gdk reconfigure and gdk restart to pick up the changes.
4. Browse to the overview page of the agent
5. In the network tab of developer tools, observe the prefix has been set in the cookie value, for example:
6. In the browser console, execute the following (substituting the correct agent ID and KAS address):

    fetch('https://gdk.test:3443/-/k8s-proxy/api/v1/namespaces', {credentials: 'include', headers: {'X-Csrf-TOKEN': document.head.querySelector('meta[name="csrf-token"]').content, 'GitLab-Agent-Id': '17'}}).then((response) => response.json()).then((data) => console.log(data));

7. The call should succeed and list the namespaces in your agent's cluster.

Related to #444679 (closed)