Format X token as integer seconds (#933)

[vineethsaivs]

Issue Description

Fixes #933.

The X (Unix timestamp) format token returned f"{dt.timestamp()}", which is a float:

>>> arrow.Arrow(2021, 3, 1, 0, 1, 47, 369226).format("X")
'1614556907.369226'      # before
>>> arrow.Arrow(2021, 3, 1, 0, 1, 47, 369226).format("XSSS")
'1614556907.369226369'   # before: stray dot + extra digits

format("X") leaked the fractional part, and composite tokens like XSSS produced a stray dot and trailing digits. As noted in the issue, this was a regression from 0.17.0, which emitted whole seconds.

Fix

Return integer seconds (int(dt.timestamp())), matching the documented X token (Unix timestamp) and the pre-1.0 behavior:

>>> ...format("X")
'1614556907'
>>> ...format("XSSS")
'1614556907369'

Parsing is unaffected (arrow.get("1614556907", "X") still round-trips, and the parser still accepts fractional timestamps).

Tests

Updated tests/test_formatter.py::test_timestamp to assert whole-second output for X (it previously asserted the float). All 623 formatter tests pass.

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Disclosure: prepared with AI assistance; reviewed by me and I can explain every line.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (4142ad8).

Additional details and impacted files

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  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
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  Hits          2315      2315           

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