refactor(calendar): moving validation logic outside of service - implements #313

[galuszkak]

First stab at this problem - did test it locally at mostly got good results. This should try solve #313

[gemini-code-assist]

Code Review

This pull request refactors the Google Calendar service to use the patch method for event updates, enabling partial updates and preserving unspecified fields. It introduces a centralized validation module, CalendarValidation.ts, which includes a new requirement that all-day workingLocation events must span exactly one day. Additionally, the update logic now supports clearing attachments by passing an empty array. Documentation and tests have been updated to reflect these changes. I have no feedback to provide.