feat: expose persistent execution counters on timer mocks

[vbreuss]

Add ITimerMock.ExecutionCount and a new IPeriodicTimerMock interface with WaitForNextTickCount so tests can observe how often a timer callback fired or WaitForNextTickAsync was entered, including activity that happened before the assertion subscribed. Counters use Interlocked.Increment and Volatile.Read, persist across Change(...) / Period mutations, and freeze on dispose. IPeriodicTimer itself is left untouched; the new IPeriodicTimerMock lives in the testing project.

[Copilot]

Pull request overview

Exposes persistent execution counters on the testing time-system mocks: adds ExecutionCount to the existing ITimerMock and introduces a new IPeriodicTimerMock (gated by FEATURE_PERIODIC_TIMER) with WaitForNextTickCount. Both counters use Interlocked.Increment/Volatile.Read, survive Change/Period mutations, and freeze on dispose because the increment sites are unreachable after disposal.

Changes:

• Added ExecutionCount to ITimerMock and incremented it in TimerMock.RunTimer after each callback invocation (including swallowed-exception cases).
• Added new IPeriodicTimerMock interface, made PeriodicTimerMock implement it, and incremented WaitForNextTickCount in WaitForNextTickAsync after the pre-notification disposal/cancellation checks.
• Updated API baseline snapshots for all target frameworks and added unit tests covering initial value, increment, cancellation-before-entry, and post-dispose freeze behaviors.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Source/Testably.Abstractions.Testing/TimeSystem/ITimerMock.cs	Adds ExecutionCount property with XML docs.
Source/Testably.Abstractions.Testing/TimeSystem/TimerMock.cs	Backs the counter with an Interlocked.Increment in RunTimer.
Source/Testably.Abstractions.Testing/TimeSystem/IPeriodicTimerMock.cs	New interface extending IPeriodicTimer with WaitForNextTickCount.
Source/Testably.Abstractions.Testing/TimeSystem/PeriodicTimerMock.cs	Implements the new interface and increments the counter on each entry.
Tests/Api/.../Expected/*.txt	Updated public-API baselines for net6/8/9/10 and netstandard2.0/2.1.
Tests/Testably.Abstractions.Testing.Tests/TimeSystem/TimerMockTests.cs	Tests for initial zero, persistence across waits, dispose freeze.
Tests/Testably.Abstractions.Testing.Tests/TimeSystem/PeriodicTimerMockTests.cs	Tests for cast, increment, pre-entry cancellation, dispose freeze.

[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

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[github-actions]

This is addressed in release v6.4.0.