Function ensure_agent_credential_at 

async fn ensure_agent_credential_at(
    agent_id: &str,
    mstore: &Arc<Store>,
    http: &Client,
    base: &str,
    user_token: impl Future<Output = Result<String, String>>,
) -> Option<String>

ensure_agent_credential’s body, with the relay base URL and the human’s token (only awaited when provisioning) injectable for tests.