[Server] Dispatch protocol events for 2026-07-28 requests

[chr-hertel]

Closes #527

StatelessProtocol now gets the builder's event dispatcher and dispatches RequestEvent, ResponseEvent and ErrorEvent the same way Protocol does - setRequest(), setResponse() and setError() are honoured, also on the streamed path.

• server/discover and subscriptions/listen still don't dispatch: both are answered by the dispatcher itself before routing, and listen has no Request class to put on the event
• NotificationEvent stays handshake-era only, since the modern era runs no notification handlers
• rejecting a request from a listener is [Server] Allow a RequestEvent listener to reject a request with a JSON-RPC error #513, not part of this

[mglaman]

Docs gap: events.md should cover two modern-era differences:

• ResponseEvent fires on each InputRequiredResult round, not just the final result. Listeners that audit completed calls need to check the result type.
• getSession() is a new in-memory session for each request. State a listener stores there is gone by the next request.