Fix replays when multiple GET /mcp streams are present

the spec uses "MAY" regarding multiple GET /mcp sse streams — it would be good to document that we assume only one SSE GET stream, and events replayed on that stream do not disambiguate between different GET streams

the root cause of this is our use of a static stream id for the GET stream(s) under the hood, `SSE_STREAM_ID = "_GET_stream"`