Function fetch_access_token 

pub async fn fetch_access_token(
    client_id: &str,
    client_secret: &str,
    refresh_token: &str,
    token_url: &str,
) -> Result<SecretString>

Exchange an OAuth2 refresh token for a short-lived access token.

token_url is parameterized for testing; production callers pass GOOGLE_TOKEN_URL.

Errors

Returns MktError::AuthError if the token endpoint rejects the exchange or the response lacks an access_token field, and MktError::Http on transport failures.