[Enhance]: ag-ui enhancement & extension, custom agent generation based on RunAgentInput.

Functional Requirements / 功能需求

#### FR-1: Context-Aware Agent Factory Interface
**English:**
- Define a functional interface `InputContextualAgentFactory` that accepts `RunAgentInput` and returns an `Agent` instance
- The interface should allow factories to access all request parameters (threadId, runId, messages, forwardedProps, tools, etc.)

**中文：**
- 定义函数式接口 `InputContextualAgentFactory`，接受 `RunAgentInput` 并返回 `Agent` 实例
- 该接口应允许工厂访问所有请求参数（threadId、runId、messages、forwardedProps、tools 等）