429 and may include Retry-After. Use exponential
backoff with jitter, preserve the original idempotency key for create retries,
and stop retrying validation (400) or permission (403) errors.
Keep polling intervals at or above the pollAfterSeconds returned by a
checkout-deposit status response. Bound concurrency, retry only safe transient
failures (429 and appropriate 503 responses), and surface a request ID when
asking support for assistance.