Function messages_probe 

async fn messages_probe(
    client: &Client,
    user_agent: &str,
    base_url: Option<&str>,
    access_token: &str,
) -> Result<HeaderMap, OauthUsageError>

One round-trip against /v1/messages with the minimal probe call: the cheapest haiku model, max_tokens 1, a tiny input - about nine billed tokens. On a 200 the response headers carry the unified windows, so the HeaderMap is the payload; a 429 keeps the windows it carried on the error; every other status classifies exactly like the windowed probe so the loader and poller treat both probes alike.