fetchJson/fetchPublicJson only reached the redirect diagnostic when the 3xx
carried an HTML body or text/html content-type; an empty-body 302/307 (the
common reverse-proxy / forward-auth shape) still resolved null through the
earlier empty-body check. http.request never follows redirects, so classify
on status first and reject every 3xx with the redirect error regardless of
body.
Pass res.headers.location through so the message says where the request was
sent, and stop blaming credentials when the Location differs from the
requested URL only by scheme or trailing slash -- that is a saved-URL
mismatch, not an authentication proxy. Drop the 404 mention from the
docstring/test: both callers reject >= 400 before this branch, so only the
2xx leg carries the endpoint-missing capability wording.
Part of #112072
10652c9345