refactor(calendar): extract validation logic into a separate layer

Context

From PR #308 review feedback by @abhipatel12:

The validations here make sense. At some point it may make sense to have a validation layer to separate all this logic to make it easier to read.

Problem

CalendarService.createEvent and updateEvent have accumulated significant inline validation logic:

• start/end dateTime vs date exclusivity checks
• eventType-specific constraints (focusTime/outOfOffice can't be all-day)
• workingLocationProperties required sub-property validation
• summary requirements based on eventType
• ISO 8601 datetime format validation
• Email format validation

This makes the methods harder to read and the validation harder to reuse or test independently.

Proposal

Extract calendar event validation into a dedicated module (e.g. CalendarValidation.ts) that:

• Consolidates all input validation for createEvent and updateEvent
• Returns structured validation errors
• Can be unit tested independently of the service
• Keeps the service methods focused on API interaction logic

Start by locating CalendarService.createEvent and updateEvent and read the existing inline checks listed in the issue. Extract them into a dedicated CalendarValidation.ts module with structured errors, then add independent unit tests covering those validations while confirming the service still handles API interaction.