Use with_child_unrestricted in pydantic_converter_v1 sample

[PardhuTI]

What

pydantic_converter_v1/worker.py carried a TODO to switch its sandbox setup to
with_child_unrestricted once temporalio/sdk-python#254 was fixed and released.
That fix has shipped, so this replaces the manual "copy the children map and
delete the datetime key" workaround with the intended one-liner.

with_child_unrestricted("datetime") is equivalent here, and slightly more
complete: for a single-element path it clears the key from access, use, and
children, whereas the old code only removed it from children.

Closes #126

Testing

pytest tests/pydantic_converter_v1/ passes (workflow round-trips Pydantic
models with datetime fields); ruff format --check, ruff check, and
mypy are clean.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}