[Feature] a2a_config配置了streaming=True，调用a2a endpoint 无法流式输出

## Problem Description
[A2AFastAPIDefaultAdapter配置了streaming=True，调用a2a endpoint 无法流式输出]

## Proposed Solution
输入这个"method": "message/stream", 希望可以流式输出
```
{
"method": "message/stream",
"id": "2d2b4dc8-8ea2-437b-888d-3aaf3a8239dc",
"jsonrpc": "2.0",
"params": {
"message": {
"role": "user",
"kind": "message",
"metadata": {
"userId": "me",
"sessionId": "test1",
"model": "2222"
},
"parts": [
{
"text": "你好.",
"kind": "text"
}
],
"messageId": "c4911b64c8404b7a8bf7200dd225b152"
},
"model": "dsdsd"

}
}
```

## Affected Component(s)
- [x] Engine
- [ ] Sandbox
- [ ] Common
- [ ] Tools
- [ ] Documentation
- [ ] Other: ___________

## Alternatives Considered

## Additional Context