fix(claude): keep a fast-mode refusal request-scoped

An account without the fast-mode entitlement answers speed:"fast" with HTTP 429
and "Usage credits are required for fast mode." The generic pipeline treats any
429 as quota exhaustion: it marks the credential Quota.Exceeded, assigns an
exponential cooldown up to 30 minutes and rotates to the next credential, so a
single request could walk and cool down the entire Claude pool over a
capability the account will never have.

Classify it through the existing RequestScopedError contract instead, which
already suppresses cooldown, rotation and retry. The status and body are then
returned verbatim: the classifier exists to stop the generic pipeline from
handling the error, not to handle it.

Detection deliberately requires both "fast mode" and a credits phrase, so a
genuine rate limit, which never mentions fast mode, keeps its cooldown. Tests
cover both observed wordings, byte-exact pass-through, and the negative cases.