Fix span()/ceil() when the next frame boundary falls in a DST gap

[uditjainstjis]

Pull Request Checklist

• 🧪 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

span() builds the ceiling by shifting the floor forward one frame with check_imaginary=True. When that next boundary does not exist because it falls in a DST gap, the shift moves it forward. America/Santiago skips 2022-09-11 00:00–01:00, so:

>>> arrow.Arrow(2022, 9, 10, 12, tzinfo="America/Santiago").ceil("day")
<Arrow [2022-09-11T01:59:59.999999-03:00]>   # before
<Arrow [2022-09-10T23:59:59.999999-04:00]>   # after

The shift now keeps the exact boundary (check_imaginary=False), and one microsecond before it is the last real instant of the frame. Boundaries that exist are unaffected.

Tests: test_span_day_before_dst_gap_at_midnight fails without the change and passes with it; test_span_day_of_dst_gap_at_midnight covers the day of the gap. Full suite: 1904 passed; pre-commit hooks pass.

Closes: #1124

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (beeda38).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1364   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
=========================================
  Hits          2315      2315           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.