open_event_stream(
self,
params: dict[str, Any],
) -> EventStreamHandleOpen an independent filtered SSE event stream.
Posts params as a SubscribeParams body to /threads/{thread_id}/stream/events.
Returns an EventStreamHandle whose events async iterator yields typed
Event dicts as the server emits them. handle.ready resolves on a 2xx
response (rejects on HTTP error or transport failure before headers).
Reconnect: pass params["since"] to filter outbound seqs server-side. The
cursor goes in the request body, not as a Last-Event-ID header.