Why : we had an issue impoting events du to CalendarEvents not yet existing while inserting the CalendarChannelAssociation due to inverted method in the service This PR refactors the calendar event import logic by - renaming - splitting utility functions for better clarity and maintainability. - adding TSDoc comments to explain the purpose and uniqueness of the `eventExternalId` field in calendar event associations Fixes #12690 --------- Co-authored-by: Charles Bochet <charles@twenty.com>