Issues with streamable http everying example:

The following issues with https://github.com/modelcontextprotocol/servers/blob/main/src/everything/transports/streamableHttp.ts

1. The documentation for EventStore["replayEventsAfter"] implies that we should be returning the streamId associated with the eventId but the code calls that lastId and then assigns it initially to eventId.
2. The InMemoryEventStore erroneously will replay all events, even those for different streams
3. Unlike the python API, the tying of EventStore["replayEventsAfter"] does not allow for returning undefined (as opposed to Python's None) when the eventId is unknown.

Start by reading src/everything/transports/streamableHttp.ts, focusing on EventStore["replayEventsAfter"] and InMemoryEventStore. Check the TypeScript behavior against the documented contract and the Python API mentioned in the issue. Done means the stream identifier, stream filtering, and unknown-event return behavior all match the stated expectations.