custom_java_impl示例中发现 返回中history-->parts中 "kind": null, 是null有问题啊。 正确的应该是"kind": "text"

示例：

client请求：
```
{
"id": "7981a512-3096-433e-a2c9-63fdfba1522a",
"jsonrpc": "2.0",
"method": "message/send",
"params": {
"id": "german-cancel-task",
"sessionId": null,
"message": {
"messageId": "b46b3722-8cd7-47d0-9914-d86895a707bf",
"kind": "message",
"role": "user",
"parts": [
{
"kind": "text",
"text": "美丽 翻译成日语",
"metadata": null
},
{
"kind": "text",
"text": "美丽 翻译成韩语",
"metadata": null
},
{
"kind": "text",
"text": "美丽 翻译成英文",
"metadata": null
}
],
"contextId": null,
"taskId": null,
"referenceTaskIds": null,
"metadata": null
},
"pushNotification": null,
"historyLength": null,
"metadata": {}
}
}

```

server返回：
```
{
"id": "7981a512-3096-433e-a2c9-63fdfba1522a",
"jsonrpc": "2.0",
"result": {
"id": "german-cancel-task",
"contextId": "62d125c0-d4ad-423c-8d8a-58af5773cc38",
"kind": "task",
"status": {
"state": "COMPLETED",
"message": null,
"timestamp": "2025-11-15T07:43:28.953500Z"
},
"artifacts": null,
"history": [
{
"messageId": "b46b3722-8cd7-47d0-9914-d86895a707bf",
"kind": "message",
"role": "user",
"parts": [
{
"kind": null,
"text": "美丽 翻译成日语",
"metadata": null
},
{
"kind": null,
"text": "美丽 翻译成韩语",
"metadata": null
},
{
"kind": null,
"text": "美丽 翻译成英文",
"metadata": null
}
],
"contextId": null,
"taskId": null,
"referenceTaskIds": null,
"metadata": null
},
{
"messageId": "d9a1d1e0-88d6-4850-8d52-268b2ff2c464",
"kind": "message",
"role": "assistant",
"parts": [
{
"kind": null,
"text": "Translate 'beautiful' to Japanese\nTranslate 'beautiful' to Korean\nTranslate 'beautiful' to English",
"metadata": null
}
],
"contextId": null,
"taskId": "german-cancel-task",
"referenceTaskIds": [
"b46b3722-8cd7-47d0-9914-d86895a707bf"
],
"metadata": null
}
],
"metadata": {}
},
"error": null
}
```

Look at the custom_java_impl example in the a2a-samples repository. The issue is that the 'kind' field in the 'parts' array of the history response is null instead of 'text'. Find the Java code that serializes the response, likely in a model class or a serializer for the message parts. Compare the request and response structures to ensure the 'kind' field is properly set. Run the example to reproduce the issue and verify the fix.

Escrita pelo modelo de indexação a partir do texto da issue.