[LindseyZ1205]

ActivityExecutionDescription.expiration_time is typed datetime | None, and documented as the time at which the activity's schedule-to-close timeout expires. When a Standalone Activity has no schedule-to-close timeout, for example when it's started with only start_to_close_timeout, the server doesn't set expiration_time, and ActivityHandle.describe() returns datetime.min, which is naive, instead of None. #1782 changed the annotation from datetime to datetime | None, but kept the datetime.min default. NexusOperationExecutionDescription.expiration_time is None in this case.

So code that follows the annotation, such as:

desc = await handle.describe()
if desc.expiration_time is not None:
    remaining = desc.expiration_time - datetime.now(timezone.utc)

raises TypeError: can't subtract offset-naive and offset-aware datetimes for an activity started with only start_to_close_timeout.

This change returns None in this case, and adds a fixed changelog fragment.

test_describe_without_schedule_to_close_timeout in tests/test_activity.py starts an activity with only start_to_close_timeout and checks that describe() returns expiration_time=None. On main, it fails with assert datetime.datetime(1, 1, 1, 0, 0) is None.

Validation, with Python 3.12, the local dev server, and the Rust bridge of the temporalio 1.34.0 wheel:

• tests/test_activity.py passes (35 tests). I didn't run the rest of the suite, since nothing else in the SDK reads this field.
• ruff check --select I and ruff format --check (ruff 0.15.22), pyright 1.1.403, basedpyright 1.34.0 and mypy --namespace-packages --check-untyped-defs pass on the changed files, and pydocstyle passes on the changed source file.