[Feature Request] Add AG UI (AgentScope Web UI) Compatibility for Java Runtime Agents

### Description

**AG-UI**, or **Agent User Interaction Protocol**, is an **open, lightweight, event-based protocol** designed to standardize how AI agents connect to user-facing applications. It facilitates a **bi-directional connection** between user applications and AI agents, enabling seamless integration and communication. AG-UI aims to simplify and enhance the interaction between users and AI systems, making it a crucial component in the development of intelligent applications.

To enable **Java-based agents to be managed and interacted with through AG UI**, we need to ensure the Java runtime:

1. Exposes a **standardized HTTP/gRPC API** that matches the AG UI backend contract
2. Produces **structured messages** in the expected schema
3. Supports **streaming output** (e.g., SSE) for real-time interaction
4. Handles **session state**, **file uploads**, and **tool approvals** consistently