fix: make half-day selection more precise

[lyind]

Fixes https://github.com/giantswarm/giantswarm/issues/27691

Summary

Single-day events which both start and end in the hour 12 (12:00 - 12:59) were erroneously interpreted and inserted/updated as whole-day time-offs in Personio (both half_day_start and half_day_end flags set).

Changes

This PR fixes the issue by improving:

1. Interpretation and normalization of Google Calendar events into internal TimeOffs objects (dates + half-days)
   • Fixes the original issue
2. Serialization of internal TimeOff objects to Personio API time-off update payload/body
   • Avoids similar issues to the original issue from occuring again (robustness)
3. Relevant tests.