Fix LocalDateTime::plusNanos() and minusNanos() within the same second

[gnutix]

Problem

LocalDateTime::plusNanos() and minusNanos() return the original value when the result stays within the same second:

LocalDateTime::parse('2000-01-01T12:34:56.5')->plusNanos(1);  // 2000-01-01T12:34:56.5, expected 2000-01-01T12:34:56.500000001
LocalDateTime::parse('2000-01-01T12:34:56')->plusNanos(1);    // 2000-01-01T12:34:56,   expected 2000-01-01T12:34:56.000000001
LocalDateTime::parse('2000-01-01T12:34:56.5')->minusNanos(1); // 2000-01-01T12:34:56.5, expected 2000-01-01T12:34:56.499999999

plusWithOverflow() reuses the current LocalTime whenever the second-of-day is unchanged, which drops the newly computed nano-of-second. Java's original compares the nano-of-day instead. The existing test cases all cross a second boundary, so they did not catch it.

The other callers (plusHours(), plusMinutes(), plusSeconds() and their minus counterparts) pass zero nanos, so the shortcut was correct for them and they are unaffected.

Change

The shortcut now also requires the nano-of-second to be unchanged. New test cases cover adding and subtracting nanos within the same second, including a whole-day jump where the second-of-day comes back equal.

🤖 Generated with Claude Code, Opus 5.5

https://claude.ai/code/session_01Fm9bvgKFasEGzpK48bQrxA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.23%. Comparing base (2cafd91) to head (6447ef4).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #136   +/-   ##
=========================================
  Coverage     99.23%   99.23%           
- Complexity     1139     1140    +1     
=========================================
  Files            48       48           
  Lines          2489     2491    +2     
=========================================
+ Hits           2470     2472    +2     
  Misses           19       19           

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[BenMorel]

Thank you, @gnutix!

[BenMorel]

Released as 0.10.4.

[gnutix]

Just so you know, it's totally okay for me if you don't release each bug fix "right away" :) If I ever have a bug fix that's urgent, I'll let you know (though I'll probably just fix it in my project with a composer patch first anyway 😄 ).

Of course, your reactivity is much appreciated, and it makes it that much nicer to contribute back to this library. 🙏

[BenMorel]

I usually prefer to not keep things unreleased, but I may group some of your upcoming fixes under the same release! 😉