Function create_auth_code 

pub async fn create_auth_code(
    base_url: &str,
    api_key: &str,
    request: &CreateAuthCodeRequest,
) -> Result<AuthCodeData, OpenRouterError>

Create an authorization code for PKCE flow (POST /auth/keys/code).

Returns an auth code ID that can be exchanged via exchange_code_for_api_key.