HTTP/SSE transport for the v3 thread-centric protocol.
Direct port of libs/sdk/src/client/stream/transport/http.ts.
ProtocolSseTransport is bound to a single thread_id at construction. Commands
go to POST /threads/{thread_id}/commands (JSON in, JSON out). Each
open_event_stream(params) opens an independent filtered SSE connection at
POST /threads/{thread_id}/stream/events with the SubscribeParams in the
request body.