Fix: shift() loses fold, changes UTC offset on ambiguous DST time

[micson-bit]

Fixes #1067

Problem

Calling .shift() on an Arrow object representing an ambiguous local time
(e.g. during a DST "fall back" transition) silently changes its UTC offset,
even when the shift amount is zero.

Root cause

In Arrow.shift(), the line:

current = self._datetime + relativedelta(**relative_kwargs)

rebuilds the datetime via relativedelta, which does not preserve the
original fold value. fold is what disambiguates repeated wall-clock
times during a DST transition. Without it, the reconstructed datetime
defaults to fold=0, causing Arrow to resolve to the wrong occurrence
of the ambiguous time.

Fix

Re-apply the original fold value immediately after the relativedelta
addition, before any further processing:

current = dateutil_tz.enfold(current, fold=self._datetime.fold)

Testing

Added test_shift_zero_seconds_preserves_offset_on_ambiguous_dst_time in
tests/test_arrow.py, which reproduces the bug from #1067. Confirmed it
fails before the fix and passes after. Full test suite (226 tests) still
passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (987bb8a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1362   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2316    +1     
  Branches       358       358           
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+ Hits          2315      2316    +1     

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