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SCM Skip: Pattern .*\[ci-skip\].* NOT matched on message: test: rename test methods and add notification service test classes

Added prefix 'test' to all test method names in UserServiceTest, PersonServiceTest,
and AdminServiceTest to improve readability and align with standard JUnit naming conventions.

Additionally, implemented and integrated two new service classes:
- UserNotificationService
- NotificationUserService

Created corresponding test classes for both services to validate the many-to-many
relationship logic between UserEntity and NotificationEntity, including methods for
adding and removing associations.

These changes enhance test coverage and maintain consistent test naming across
all service layers.