/**
 * Extract the hostname-only FQDN from a host-like string.
 *
 * `Cdn.host` may arrive from /decision or from the setExternalProviders
 * overlay as a bare hostname, a host:port pair, a host/path fragment or a
 * full URL. The /cdn wire contract wants only the FQDN (no scheme, port,
 * userinfo, path, query or fragment), so this strips everything else.
 *
 * Returns undefined for empty or non-hostname-shaped input (including
 * bracketed IPv6 literals, which are not FQDNs) so callers can leave the
 * wire field out instead of sending garbage. Never throws: a malformed
 * host must not break the /cdn ping. Pure string parsing on purpose, with
 * no reliance on the URL global, so the duplicated ES5/NWF/RN balancer
 * copies behave identically on legacy runtimes.
 */
export declare function extractHostname(value?: string | null): string | undefined;