feat: add callback for PeriodicTimer

[vbreuss]

This PR adds an extensibility/notification callback that fires when a mocked IPeriodicTimer is waiting for its next tick, aligning PeriodicTimer with the existing MockTimeSystem.On.* notification pattern.

Changes:

• Introduces IPeriodicTimerNotificationHandler and exposes it via INotificationHandler.PeriodicTimer (feature-gated).
• Wires PeriodicTimerMock.WaitForNextTickAsync to invoke the new notification via NotificationHandler.
• Adds test coverage for callback invocation, disposal behavior, multiple callbacks, and predicate filtering.

[Copilot]

Pull request overview

This PR adds an extensibility/notification callback that fires when a mocked IPeriodicTimer is waiting for its next tick, aligning PeriodicTimer with the existing MockTimeSystem.On.* notification pattern.

Changes:

• Introduces IPeriodicTimerNotificationHandler and exposes it via INotificationHandler.PeriodicTimer (feature-gated).
• Wires PeriodicTimerMock.WaitForNextTickAsync to invoke the new notification via NotificationHandler.
• Adds test coverage for callback invocation, disposal behavior, multiple callbacks, and predicate filtering.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Tests/Testably.Abstractions.Testing.Tests/TimeSystem/PeriodicTimerMockTests.cs	Adds tests validating the callback behavior from the PeriodicTimerMock perspective.
Tests/Testably.Abstractions.Testing.Tests/TimeSystem/NotificationHandlerTests.cs	Adds tests validating notification registration/disposal, multiple callbacks, and predicate filtering.
Source/Testably.Abstractions.Testing/TimeSystem/PeriodicTimerMock.cs	Invokes notification when WaitForNextTickAsync is called; stores callback handler.
Source/Testably.Abstractions.Testing/TimeSystem/PeriodicTimerFactoryMock.cs	Passes the callback handler into created PeriodicTimerMock instances.
Source/Testably.Abstractions.Testing/TimeSystem/NotificationHandler.cs	Implements the new periodic-timer notification registration + invocation.
Source/Testably.Abstractions.Testing/TimeSystem/IPeriodicTimerNotificationHandler.cs	Adds the public interface describing the periodic timer notifications (feature-gated).
Source/Testably.Abstractions.Testing/TimeSystem/INotificationHandler.cs	Exposes PeriodicTimer notifications off MockTimeSystem.On (feature-gated).
Source/Testably.Abstractions.Testing/MockTimeSystem.cs	Wires NotificationHandler into the periodic timer factory so notifications can be raised.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

This is addressed in release v6.2.0.