Function is_loopback_host 

pub(crate) fn is_loopback_host(host: &str) -> bool

Returns true if host names the loopback interface.

Recognizes localhost, any address in 127.0.0.0/8, and ::1 (with or without URL-style brackets). Used by the SQL TLS defaults and by mz-deploy mcp to pick http:// vs https://.