The AI chat UI previously collapsed every non-403/503 failure into a
generic "could not respond" message, hiding real provider errors such as
OpenRouter HTTP 402 "requires more credits". The backend already forwards
the real "<status>: <message>" via pipeUIMessageStreamToResponse onError,
so the fix is client-side.
- describeError now returns the provider message verbatim for any error
that is not one of our own gating responses, so 402 (credits), 429
(rate limit) and similar causes are visible to the user.
- Match gating responses by the NestJS JSON "statusCode" field instead of
loose substring/word checks, so a provider message that merely contains
"403"/"503"/"disabled" is no longer misclassified and hidden.
- Add a providerDetail() helper that filters empty text and the opaque
"An error occurred." / "Internal server error" placeholders, falling
back to the generic message only then.
No backend changes; no new i18n keys.
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