Adds handling for half day OOO requests

[tamaradement]

PR Description

• Adds support for creating Google Calendar events from half-day Clockify OOO requests.
• Refactors the all-day and half-day time calculations into dedicated handleAllDay and handleHalfDay helpers.
• Adds unit tests for the new and refactored time-handling logic.

Test Steps

• Ran the test suite successfully.
• Exercised the pipeline with first-half, second-half, and full-day OOO requests.
• Confirmed that findClockifyEvents locates existing half-day events.