isRetryable

Whether re-sending the failed entry could plausibly succeed.

Terminal errors caused by the request itself — an unmet precondition, an unsupported or oversized file, a missing target, or a disabled/unprocessable upload — return false, since re-sending the same content will fail the same way. Transient errors (a general server error or a rate limit that clears with time) return true. Use this to avoid presenting a futile retry.