Preserve microsecond precision in is_between

[FanWu-ai]

Pull Request Checklist

• 🧪 Added tests for changed code.
• 🛠️ All tests pass locally on Python 3.12.14.
• 🧹 All configured linting checks pass locally.
• 📚 Updated documentation for changed code. No public API change.
• ⏩ Code is up-to-date with master.

Description of Changes

is_between converts all three dates to floating-point timestamps. Distinct microseconds can collapse for early or late years. For example, a date one microsecond after Arrow(2500, 1, 1) is incorrectly considered outside the open interval ending two microseconds after it. Inclusive bounds can also admit a target outside an interval when its timestamp rounds to an endpoint.

Compare exact timedeltas relative to a fixed UTC-aware epoch instead. This preserves microseconds and UTC-instant ordering, including DST folds, without constructing out-of-range UTC datetimes at year 1 or 9999.

Add 28 regression/compatibility cases covering six years, all four bounds, mixed offsets, UTC datetime boundaries, and both DST fold values.

Validation

• Untouched base: 1,904 tests passed
• New tests on original source: 19 failed, 9 compatibility cases passed
• Final full suite: 1,932 passed, no skips or xfails; 99.93% coverage
• All 23 configured pre-commit hooks, including Black, Flake8 and mypy, passed
• doc8 and warnings-as-errors Sphinx HTML build passed
• git diff --check passed
• Independent integer-reference checks: 40,000 comparisons across years 1–9999, offsets ±23:59 and every bounds mode passed

Tested on Linux with Python 3.12.14. No hosted Python/OS matrix result is claimed yet. Prepared with OpenAI Codex assistance.