Resolve VSN-1870 "Feature/expose getPassengerDetails endpoint for trip addPoint flow"
- Added new getPassengerDetails endpoint to fetch passenger establishment / representative information for addPoint
- Added request/response contracts: PassengerDetailsRequest, PassengerDetailsResponse, WeekTypeScheduleDTO, WeekdayPeriodDTO
- Implemented business logic in PassengerServiceImpl to resolve the unique establishment and representative covering all requested (day, period) slots
- Added CollectionUtils.safeList() for null-safe list handling
- Added EnumConverters (toDomainDay / toPeriodOfDay) for enum mapping between transport enums Closes VSN-1870