Function relay_server_name 

pub fn relay_server_name(host: &str) -> Result<ServerName<'static>>

Parse relay_host the way the connection path consumes it: as the outer TLS server name, and as the host of the wss:// registration URI.

Enrollment calls this before persisting a claimed host, so a value either consumer would reject is refused up front instead of surfacing as invalid relay host at registration, after the claim reported success.