Clarify tool event behavior of fronted and backend

The current AG-UI tool calling protocol mixes two distinct modes:

1. The tool call is actually executed by the backend/remote side, and events are synchronized to the frontend for display purposes only.
2. The tool call is executed by the frontend/client side, with results returned to the backend/remote side — similar to Human-in-the-Loop (HITL).

However, the current protocol specification does not clearly distinguish between these two modes. How should developers differentiate between them? The documentation should be improved to clarify this.

Additionally, for function calls, parameters should support the `dict` type for greater clarity and precision. Relying solely on `string` types introduces unnecessary serialization/deserialization overhead.