parser: round microsecond half-up when truncated value has trailing non-zero digits

[HrachShah]

Bug

arrow.parser.DateTimeParser rounds the 'S' sub-second token to 6 digits using only the 7th digit. When the 7th digit is 5, it applies round-half-to-even (banker's rounding) based purely on the parity of the 6th digit, ignoring everything that follows.

A value like .789124501 therefore rounds to 789124 (banker's: 4 is even), but the true value is strictly above the half-way point between 789124 and 789125 and should round to 789125. The same is true for .789124511, .789123550001, and any other input where the 7th digit is 5 and a non-zero digit trails it.

Repro

>>> import arrow
>>> arrow.get(2013-02-03T04:05:06.789124501).format(S)
'789124'   # wrong, should be '789125'
>>> arrow.get(2013-02-03T04:05:06.789123500).format(S)
'789124'   # correct (banker's: 3 is odd, round up; trailing zeros make it exactly half-way)

Fix

Restrict the banker's-rounding branch to the case where the 7th digit is 5 and every following digit is zero. If any non-zero digit trails the 5, the truncated 6-digit value sits strictly above the midpoint, so round up regardless of parity:

if seventh_digit > 5:
    rounding = 1
elif seventh_digit < 5:
    rounding = 0
else:  # exactly 5
    if any(ch != "0" for ch in value[7:]):
        rounding = 1   # strictly above the midpoint
    else:
        rounding = int(value[5]) % 2   # banker's rounding

Tests

• Updated test_parse_subsecond_rounding (the "round half-down" / "round half-up" cases were pinning the bug).
• Updated test_gnu_date (date -Ins output that has 9 sub-second digits is now rounded correctly).
• Added test_YYYY_MM_DDTHH_mm_ss_S_round_half_up covering all four corners: banker's with even 6th / trailing zeros, banker's with odd 6th / trailing zeros, strict half-up with non-zero trailing, and strict above/below the half-way point.

Full test suite: 1905 passed, 1 skipped (up from 1904 — the new test). The 2 pre-existing errors in tests/utils.py (assertion helpers being collected as test items) are unrelated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (0198620).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2317    +2     
  Branches       358       359    +1     
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+ Hits          2315      2317    +2     

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