feat(mobile): Write test for notification_domain_layer

[duressaJemal]

Description:
This pull request includes tests for the data layer of the notifications feature. Specifically, it includes the following test files:

• notification_test.dart: This file tests the Notification entity to ensure that it is correctly constructed and its properties are set and retrieved correctly.
• get_notification_test.dart: This file tests the GetNotification use case to ensure that it can successfully retrieve a list of notifications from the repository.
• send_is_read_test.dart: This file tests the SendIsRead use case to ensure that it can correctly send the "is read" flag to the remote API.

The tests cover various scenarios, including:

• Testing the construction and property access of the Notification entity.
• Testing the GetNotification use case to ensure that it can correctly handle successful and failed requests to the repository.
• Testing the SendIsRead use case to ensure that it can correctly handle successful and failed requests to the remote API.

Checklist:

• Added tests for the Notification entity in notification_entity_test.dart
• Added tests for the GetNotification use case in get_notification_test.dart
• Added tests for the SendIsRead use case in send_is_read_test.dart
• Ensured that all tests pass.