WebhooksService.create() rejected obviously private, loopback, or
link-local URLs (incl. cloud metadata 169.254.169.254 and the
decimal/octal/hex IPv4 shorthand encodings the WHATWG URL parser
normalizes) so an admin-configured webhook can no longer be used to
proxy requests into internal infrastructure. Registration-time only,
no DNS resolution (DNS-rebinding is out of scope for this check).