Add Mockito tests alongside lambda-based notifier test

- Keep lambda-based test as a simple delegation example
- Add Mockito mocks for interaction verification
- Verify no interactions on invalid input
- Capture and assert notification arguments
- Add input validation to NotificationService

Add parameterized tests for UserRecord validation

- Validate name, email and age using parameterized tests
- Cover multiple invalid combinations with CsvSource
- Align tests with stricter email validation rules

Add CI status badge to README

Add pattern matching with instanceof and modern switch

Introduce User record with validation and normalization

Introduce Optional usage for repository lookups

Add Stream-based implementations for collections with tests

Simplify notifier test using a lambda

Add notifier interface and service abstraction

Add User domain model with encapsulation and identity

- Implement immutable User with validation
- Add email normalization and validation
- Define equality based on email
- Implement hashCode and toString
- Add comprehensive unit tests