[Bug] Update Pydantic converter example to reflect `with_child_unrestricted` changes

What are you really trying to do?

I want to use Pydantic and datetime objects as input/outputs/signals/queries to workflows and activities.

Describe the bug

The recommended reference example for the Pydantic converter creates a new sandbox to work around a bug in Pydantic.

There is a TODO in the code below:

https://github.com/temporalio/samples-python/blob/main/pydantic_converter/worker.py

that says:

TODO(cretz): Use with_child_unrestricted when https://github.com/temporalio/sdk-python/issues/254 is fixed and released

The issue in question has now been merged but the example hasn't been updated.

Minimal Reproduction

https://github.com/temporalio/samples-python/blob/main/pydantic_converter/worker.py

Environment/Versions

n/a

Additional context

n/a

Thanks!

打开 pydantic_converter/worker.py，阅读关于在 sdk-python issue 254 修复并发布后使用 with_child_unrestricted 的 TODO。更新参考示例以反映这一变更，然后验证 Pydantic 转换器示例不再依赖 issue 中描述的 workaround。