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When FetchUsageWithOptions receives a non-200 response from the
Anthropic API (e.g. 429 Rate Limited, 401 Unauthorized), the error
was wrapped with fmt.Errorf which infraerrors.FromError cannot
recognize, causing a generic "internal error" message with no details.

Replace fmt.Errorf with infraerrors.New(500, "UPSTREAM_ERROR", msg)
so the upstream error details (status code + body) are included in
the 500 response message. The HTTP status remains 500 to avoid
interfering with frontend auth routing (e.g. 401 would trigger
JWT expiry redirect).