startQuery method
Starts a query on the extension. params carries streamId, prompt,
and (optionally) options (the serialized Options wire JSON).
Returns once the query has been started; the resulting SDKMessages
arrive asynchronously on chunks, not as the result of this call.
Implementation
@override
Future<void> startQuery(Map<String, dynamic> params) async {
await client.sendRequest('agentSdk.queryVce', params);
}