[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!

Mở pydantic_converter/worker.py và đọc TODO về việc sử dụng with_child_unrestricted sau khi sdk-python issue 254 được sửa và phát hành. Cập nhật ví dụ tham chiếu để phản ánh thay đổi đó, sau đó xác minh rằng ví dụ Pydantic converter không còn phụ thuộc vào workaround được mô tả trong issue.