turn_api_error enters the fallback walk only for ``retryable=False`` verdicts outside the
retryable-client reasons; the built-in terminal verdicts pin retryable=False while the
rate-limit family stays retryable and cascades after backoff. A ``classify_api_error`` hook
returning ``{"reason": "billing", "should_fallback": True}`` therefore retried the dead
route instead of cascading (the #116408 test passed only because its hook also set
retryable=False). Default retryable to False for such verdicts, leaving rate-limit reasons on
the built-in retry-then-fallback shape; ``RETRYABLE_CLIENT_REASONS`` moves next to the
verdict table so both modules read one set. Contract documented in the plugin guide.
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