Function post_jsonrpc 

async fn post_jsonrpc(
    client: &Client,
    url: &str,
    req: &Value,
) -> Result<Value, ProbeResult>

POSTs one JSON-RPC request and returns the first JSON object in the response body — handling both a plain application/json body and a Streamable HTTP text/event-stream framing (event: message\ndata: {...}\n\n), since a probe only needs the first frame either way.