[Bug]: AGUI protocol fails to correctly serialize LLM responses when using OllamaChatModel

**Describe the bug**
When using the official AGUI example with `OllamaChatModel`, calling the `/agui/run` endpoint returns an error response: `messageId cannot be null`.

**To Reproduce**
For easier debugging and to inspect raw requests/responses, I modified the `stream` parameter in the `streamWithHttpClient` call inside `OllamaChatModel` from `true` to `false`. Specifically, in the `doStream` method:

```java
@Override
protected Flux doStream(
List messages, List tools, GenerateOptions options) {
return streamWithHttpClient(
messages,
tools,
options.getToolChoice(),
OllamaOptions.fromGenerateOptions(options),
false);
}
```

When sending a request as per the example, debugging shows that the raw response from Ollama **does not contain an `id` field**:

As a result, `AguiEvent` fails to extract a `messageId`, causing the client-side to throw an error:

**Expected behavior**
The system should return the LLM’s response content successfully.

**Environment**

- AgentScope-Java Version: 1.0.8
- Java Version: 17
- OS: windows

**Additional context**
When using `OpenAIChatModel` under identical conditions, everything works as expected.

In contrast, when using `JdkHttpTransport` with OpenAI, the actual HTTP response **does include an `id` field**:

Thus, `AguiEvent` correctly obtains the `messageId`, and the client receives the expected response: