Implement tests for CalendarViewModel

[othbcq]

Description:

The CalendarViewModel currently lacks sufficient unit tests to verify its core functionalities. Implement comprehensive unit tests to ensure the model behaves as expected, particularly around event filtering, sorting, and handling recurring and multi-day events.

Tasks:

• Write unit tests for filtering and sorting events.
• Test the handling of recurring events, ensuring they are properly processed and displayed.
• Test multi-day event handling to confirm accurate representation across consecutive days.
• Ensure edge cases are covered (e.g., events spanning the start or end of the testing period).

[othbcq]

Closing this issue to combine with #39

After reviewing both tasks, I’ve decided to combine this issue with issue #39 to streamline the development process. Since both issues relate to the CalendarViewModel and the Calendar feature, it makes sense to address them together. By merging the two, I’ll be able to manage everything in one pull request instead of two, ensuring a more efficient and cohesive update for the calendar functionality.