Variable httpFetchConst

HTTP fetch for the Node build. Uses undici's own fetch (not the global one) so fetch and the buildDispatcher dispatcher share one undici version, whatever undici the Node runtime bundles (e.g. Node 26 ships undici 8, incompatible with an undici 6 dispatcher). A holder object, not a bare export, so tests can swap fetch (an ESM export can't be reassigned, an object property can). The cast aligns undici's fetch type with the global one.