The connection-test route mapped a provider 5xx to a 502, which a reverse
proxy rewrites to its own HTML error page; the browser's res.json() then
threw "Unexpected token '<'". Every provider-call failure now returns
HTTP 200 with a secret-free { ok:false, providerType, reason }. Reasons
are categorised: credential/auth (401/403, or a 5xx whose body signals an
invalidated/expired session), rate-limit (429), provider server error
(other 5xx) and network/timeout.
The empty-override "test my saved config" path now resolves through the
same provider chain generation uses, instead of the legacy codex->admin
probe that could test a different provider than the one generation runs.
The chain runner already advances past an auth-class failure to the next
enabled provider; add explicit coverage for that on the raw generation
path (401 -> next succeeds, fallback count incremented).