SEP: Context update mechanism

[idosal]

An app-generated event to update the agent's context without requiring a follow-up action (e.g., a prompt) has many use cases (for example see SEP PR by @PederHP, @tschei-di, and others).

That’s the original intent behind notify, but there are semantic differences between MCP-UI’s notify (which updates the context) and the proposed notifications/message (which MCP describes as general logging).

We should decide how to handle it (e.g., use a different event, define different semantics, or use an alternative mechanism).