Restore clipped day in Arrow.range() for plural frame names

[Str0k]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

Description of Changes

Arrow.range() accepts plural frame names ("months", "quarters", "years") through _get_frames(), and they iterate over the same steps as the singular names. But the end-of-month "restore the clipped day" logic compares the raw frame argument against ["month", "quarter", "year"], so it never fires for the plural spellings. After passing through a shorter month, the day stays clipped for the rest of the range:

>>> [a.format("YYYY-MM-DD") for a in arrow.Arrow.range("month", datetime(2015, 1, 31), limit=4)]
['2015-01-31', '2015-02-28', '2015-03-31', '2015-04-30']
>>> [a.format("YYYY-MM-DD") for a in arrow.Arrow.range("months", datetime(2015, 1, 31), limit=4)]
['2015-01-31', '2015-02-28', '2015-03-28', '2015-04-28']   # before this change

span_range() forwards frame to range() unchanged, so it was affected in the same way.

The fix keeps the normalized frame name that _get_frames() already returns and uses it for the comparison, so "month"/"months", "quarter"/"quarters" and "year"/"years" behave identically. The existing singular-name regression tests (e.g. #842) are unchanged.

Tests

Added TestArrowRange.test_range_plural_frames_restore_clipped_day, parametrized over "months", "quarters" and "years" using the same dates as the existing singular-name tests. All three cases fail on master and pass with this change.

Locally on CPython 3.12: pytest tests → 1903 passed, 2 skipped. black --check, flake8 and isort --check-only are clean on the changed files. mypy arrow/arrow.py reports 3 errors in humanize() (lines ~1268–1296) that are identical with and without this change, so they come from my newer mypy version rather than from this PR. I did not run the full tox matrix, only the pytest suite on 3.12 and the linters above.

No documentation change: the docstring already lists these frames as supported, and this makes the plural forms behave as documented.

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This change was prepared with AI assistance (Claude Code). I reproduced the bug on master, reviewed the diff and ran the tests and linters described above.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (64ff518).

Additional details and impacted files

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