Admin-only setter for control plane secrets stored in auth.json.
This endpoint currently supports callbackToken only.
Non-empty values are trimmed and persisted. An empty string clears the
saved token. The response never returns the raw token; it only reports
whether a callback token is configured after the update.
Session cookie authentication
Callback token to persist for control plane connector requests. Send an empty string to clear the saved token.
"cpt_xxx"