utils.fetch is a synchronous, read-only GET or HEAD to the channel Base URL
host or the plugin's allowedHosts, through the channel proxy used for
polling. It is available in buildSubmitRequest, the query and batch build
and parse hooks, and buildContentRequest; every other hook throws. Each hook
call gets at most 4 fetches, 15 s per fetch, 30 s in total and 1 MiB per
body. Redirects are returned, not followed. responseType "bytes" returns an
ArrayBuffer for binary data such as a media file header.
While a fetch waits, the runtime gives up its execution slot and the
watchdog pauses. BuildContentRequest now receives the request context, and
the media proxy shares the plugin request header rules.
