The JSON guard in fetchJson/fetchPublicJson blamed every HTML reply on a
missing backend endpoint. A 3xx HTML body is an access proxy redirecting
to its login page: the endpoint exists, the credentials never arrived.
Besides misleading the user, the wording is the capability signal that
isMissingHealthEndpointError and the renderer's gateway-rpc predicate key
on, so an auth redirect was silently classified as "endpoint missing" and
routed onto compatibility paths instead of surfacing as an error.
Build the error in api-transport.ts (where the sibling httpStatusError
lives) and pick the hint by status: 3xx names the redirect and points at
the saved token/extra headers; everything else keeps the endpoint-missing
wording the predicates rely on.
Part of #112072
teknium1
47c3683f23