Convert notification settings manager to use atomic reads/writes

[rbev]

This pull request refactors the notifications settings persistence layer to simplify its interface and implementation, removing the INotificationsManager abstraction in favor of direct LoadSettings and SaveSettings methods on INotificationsDataStore. It also updates the tests to match the new API and adds cancellation support. The changes affect both EFCore and RavenDB persistence implementations, as well as their respective test suites.

Persistence Layer Refactoring

• Removed the INotificationsManager abstraction and its implementations (NotificationsManager for both EFCore and RavenDB), replacing them with direct LoadSettings and SaveSettings methods on INotificationsDataStore. This simplifies how notification settings are loaded and saved throughout the codebase. [1] [2] [3] [4]
• Updated the EFCore and RavenDB implementations of INotificationsDataStore to provide the new methods, ensuring settings are loaded and saved directly via the data store. [1] [2]

Test Suite Updates

• Refactored all affected tests to use the new LoadSettings and SaveSettings API instead of the removed manager abstraction, and added tests to verify cancellation token propagation. [1] [2]
• Added helper methods in test context classes to check for the existence of notification settings in both EFCore and RavenDB implementations. [1] [2]

Application Code Updates

• Updated the SetupNotificationSettings class in acceptance tests to use the new LoadSettings and SaveSettings methods directly, reflecting the removal of the manager abstraction.