perf: drop unnecessary none-value fields in tool call loop

[Soulter]

Modifications / 改动点

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。/ If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
• 👀 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。/ My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
• 🤓 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到了 requirements.txt 和 pyproject.toml 文件相应位置。/ I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
• 😮 我的更改没有引入恶意代码。/ My changes do not introduce malicious code.

Summary by Sourcery

增强功能：

• 为助手消息添加自定义序列化器，当工具调用字段未设置时将其省略，以减少负载大小和开销。

Original summary in English

Summary by Sourcery

Enhancements:

• Add a custom serializer for assistant messages that drops tool call fields when they are not set to reduce payload size and overhead.

[sourcery-ai]

Hey - 我在这里给出了一些整体性的反馈：

• 自定义的 serialize 方法目前只会去掉 tool_calls 和 tool_call_id；如果你的意图是为了性能普遍避免发送值为 None 的字段，可以考虑配置 exclude_none，或者遍历所有字段来删除 None 值，而不是只对这两个字段做硬编码处理。
• 如果基础的 Message 类定义了自己的 model_serializer，当前的重写会绕过任何共享的序列化逻辑；可以考虑显式地委托给父类，或者统一整合相关逻辑，以避免出现行为上的分歧。

给 AI 代理的提示（Prompt for AI Agents）

Please address the comments from this code review:

## Overall Comments
- 自定义的 `serialize` 方法目前只会去掉 `tool_calls` 和 `tool_call_id`；如果你的意图是为了性能普遍避免发送值为 `None` 的字段，可以考虑配置 `exclude_none`，或者遍历所有字段来删除 `None` 值，而不是只对这两个字段做硬编码处理。
- 如果基础的 `Message` 类定义了自己的 `model_serializer`，当前的重写会绕过任何共享的序列化逻辑；可以考虑显式地委托给父类，或者统一整合相关逻辑，以避免出现行为上的分歧。

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Original comment in English

Hey - I've left some high level feedback:

• The custom serialize method only strips tool_calls and tool_call_id; if the intent is to generally avoid sending None fields for performance, consider configuring exclude_none or iterating over all fields to drop None values instead of hardcoding just these two.
• If the base Message class defines its own model_serializer, this override will bypass any shared serialization behavior; consider explicitly delegating to the parent or consolidating the logic to avoid divergence.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The custom `serialize` method only strips `tool_calls` and `tool_call_id`; if the intent is to generally avoid sending `None` fields for performance, consider configuring `exclude_none` or iterating over all fields to drop `None` values instead of hardcoding just these two.
- If the base `Message` class defines its own `model_serializer`, this override will bypass any shared serialization behavior; consider explicitly delegating to the parent or consolidating the logic to avoid divergence.

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