Function www_authenticate_challenge 

pub(crate) fn www_authenticate_challenge(
    resource_metadata_url: &str,
    scope: &str,
    error: Option<(&str, &str)>,
) -> String

The WWW-Authenticate value of a 401 or 403 from the MCP server: where the client finds the authorization server (RFC 9728) and which scopes to ask for. error is the RFC 6750 error code and description, absent when no token was sent.