fix(harness): use UTC as fallback timezone in EnvironmentResolver

[caoergou]

What and Why

EnvironmentContext.today_date is documented as returning a timezone-aware datetime (docstring: "Timezone-aware datetime on the computer"), and the inline comment in EnvironmentResolver.resolve() reads "Parse into a timezone-aware datetime."

However, the except ValueError fallback path produced a naive datetime (no tzinfo), suppressing the Ruff DTZ007 warning with # noqa rather than fixing the root cause:

# Before
except ValueError:
    now = datetime.strptime(raw_dt[:19], "%Y-%m-%dT%H:%M:%S")  # noqa: DTZ007

The existing test even asserted env.today_date.tzinfo is None, directly contradicting the documented contract. Any downstream code that performs timezone-aware arithmetic or comparisons on today_date would raise a TypeError on hosts where date outputs no UTC offset.

Fix

Attach UTC explicitly in the fallback path so the tzinfo guarantee is always upheld:

# After
from datetime import UTC, datetime

except ValueError:
    now = datetime.strptime(raw_dt[:19], "%Y-%m-%dT%H:%M:%S").replace(tzinfo=UTC)

This removes the # noqa: DTZ007 suppression (the warning no longer applies) and aligns the test assertion with the documented contract.

Changes

• uniharness/harness/environment.py: import UTC; replace naive fallback with .replace(tzinfo=UTC); remove # noqa: DTZ007
• tests/unit_tests/harness/test_environment.py: update docstring and assertion in test_datetime_without_timezone_fallback

Test Plan

cd libs/uniharness
make lint && make test

Key test: tests/unit_tests/harness/test_environment.py::TestResolve::test_datetime_without_timezone_fallback — now asserts tzinfo is not None.

Fixes #26