fix(util): Decode partition dates as UTC regardless of the host timezone

[RaphaelFakhri]

decode_part_str parses the partition date with datetime.strptime(...), which returns a naive datetime, and then calls .astimezone(UTC). Python treats a naive datetime passed to astimezone() as local time, so on a host whose TZ is east of UTC the decoded date moves to the previous day. For example, with TZ=Asia/Tokyo the partition (90,'2026-10-12') (a Monday) decodes as 2026-10-11 15:00 UTC (a Sunday).

Every caller assumes the decoded date is the partition's Monday:

• snuba cleanup: filter_stale_partitions computes the partition's last day as the end of the week, which becomes the Sunday before the partition starts. Partitions are dropped 7 days early, while they still hold data inside the retention window.
• Lightweight deletes: _get_partition_dates returns 2026-10-11, and the split delete filters on toMonday(timestamp) = '2026-10-11'. That condition matches no rows, so the delete removes nothing for that partition and the partition is still recorded as done.
• snuba optimize --before: the cutoff comparison is off by a week.

This change uses .replace(tzinfo=UTC) so the date is always the partition's UTC date. Hosts running with TZ=UTC (or a zone west of UTC) see no change.

The new test in tests/test_util.py sets TZ=Asia/Tokyo and checks that decode_part_str returns the Monday at midnight UTC. It fails before this change and passes after.

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Fixes #8561

[MeredithAnya]

I'd use a context manager here - something like

@pytest.fixture
  def local_timezone(monkeypatch: pytest.MonkeyPatch) -> Generator[None]:
      with monkeypatch.context() as m:
          m.setenv("TZ", "Asia/Tokyo")
          time.tzset()
          yield
      time.tzset()

avoids calling monkeypatch.undo()