Real SystemOneClient implementation over Req.
Retries 429, 529, 502, 503 and 504 responses and transport errors up to :max_retries
(default 2), waiting for Retry-After when the server sends one and otherwise backing
off exponentially from :retry_delay_ms (default 250). :receive_timeout defaults to
2 s. Never raises for network conditions and never puts the key into a returned term or
a telemetry event.
Telemetry: [:system_one_client, :request, :start | :stop | :exception] with
provider, model and question_count; stop adds status (:ok | :error),
latency_ms, and on error an error kind. State, questions and keys are never included.