Messaging API - Fix double processing & align transports

[linear]

The current messaging API has several architectural issues:

1. Double/triple - createMessage() always emits to internalMessageBus, even for HTTP/SSE which also call elizaOS.handleMessage() directly

2. Dead code - handleWebSocketMode() in response-handlers does nothinG useful

3. Duplication - Sessions and Channels duplicate the same sending logiC