Fix tzrange default DST offset when standard offset is omitted - #1614

Summary of changes

tzrange("STD", dstabbr="DST") currently loses the documented default one-hour DST offset when the optional standard offset is omitted. Use the already-normalized standard offset when computing that default, and cover omitted, None, explicit-zero, and explicit-DST-offset cases, plus checks around both default transitions.

With datetime(2014, 7, 15, 12, tzinfo=tzrange("STD", dstabbr="DST")), the current result is 2014-07-15T12:00:00+00:00 with zero DST; the documented default is 2014-07-15T12:00:00+01:00 with one hour of DST. The equivalent call with an explicit zero standard offset already produces the latter result. This PR is the bug report; there is no separate issue to close.

Tests

• Before the fix: 5 new cases failed, 11 passed.
• After the fix: 16 targeted cases passed.
• TZ=America/New_York .venv/bin/python -m pytest -q -p no:cacheprovider tests --tb=short: 2,047 passed, 47 skipped, 17 xfailed (Python 3.13.12, macOS arm64).
• Darker/isort and git diff --check: passed.

Other Python versions and platforms remain for hosted CI.

Pull Request Checklist

• Changes have tests
• Authors have been added to AUTHORS.md
• News fragment added in changelog.d/1614.bugfix.rst

AI assistance was used to prepare the implementation and tests.