Function build_authorization_url 

pub fn build_authorization_url(
    authorize_url: &str,
    client_id: &str,
    redirect_uri: &str,
    state: &str,
    scopes: &[String],
    code_challenge: &str,
    login_hint: Option<&str>,
) -> String

Build the browser authorization URL (RFC 8252 style; the user opens it externally) with PKCE S256, state, scopes, and an optional login hint.