This endpoint allows a partner to fetch the API keys (ID, label, creation date and key prefix, never the secret) of one of its child partners from the Partner Portal. A portal user who was removed from the partner or deactivated is refused with INVALID_ACCESS even with a still-valid token. Partner Portal session only: the request must carry the Partner Portal bearer token of a user with the Admin role, and the partner must have both AccountCredentialsAccess and PartnerCreationEnabled. The target must be an Active child partner of the caller. Returns RESOURCE_NOT_FOUND when it is not.