Function read_startup 

pub async fn read_startup<R>(
    stream: &mut R,
) -> Result<FrontendMessage, PgWireError>
where
    R: AsyncRead + Unpin,

Read the initial StartupMessage (or SSL/GSS request). The startup frame has no type byte — just a length prefix followed by the payload. Returns either a decoded Startup/SSL/GSS message or an error.