feat: support async wait handlers

[vbreuss]

This pull request adds async wait handler support to the IAwaitableCallback<TValue> interface by introducing a new WaitAsync method. The changes enable asynchronous waiting for callbacks using modern async/await patterns with cancellation token support.

Key Changes:

• Added WaitAsync method to IAwaitableCallback<TValue> for asynchronous waiting with cancellation support
• Refactored the interface to change the generic constraint from out TValue (covariant) to TValue (invariant) to support returning arrays
• Implemented channel-based async infrastructure using System.Threading.Channels for efficient value buffering
• Added comprehensive test coverage for the new WaitAsync functionality
• Updated API surface files across all target frameworks (.NET 6.0, 8.0, 9.0, 10.0, netstandard2.0, netstandard2.1)

---

• Fixes MockFileSystem.File.Move raises FileSystemWatcher event with delay #2 #938

[Copilot]

Pull request overview

This pull request adds async wait handler support to the IAwaitableCallback<TValue> interface by introducing a new WaitAsync method. The changes enable asynchronous waiting for callbacks using modern async/await patterns with cancellation token support.

Changes:

• Added WaitAsync method to IAwaitableCallback<TValue> for asynchronous waiting with cancellation support
• Refactored the interface to change the generic constraint from out TValue (covariant) to TValue (invariant) to support returning arrays
• Marked the old ExecuteWhileWaiting methods as obsolete, encouraging users to execute callbacks before calling Wait/WaitAsync
• Implemented channel-based async infrastructure using System.Threading.Channels for efficient value buffering
• Added comprehensive test coverage for the new WaitAsync functionality
• Updated API surface files across all target frameworks (.NET 6.0, 8.0, 9.0, 10.0, netstandard2.0, netstandard2.1)

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
Source/Testably.Abstractions.Testing/IAwaitableCallback.cs	Added new WaitAsync method and updated Wait method signatures; changed generic constraint from covariant to invariant
Source/Testably.Abstractions.Testing/Notification.cs	Implemented channel-based async infrastructure with WaitAsync support; marked ExecuteWhileWaiting as obsolete
Source/Testably.Abstractions.Testing/AwaitableCallbackExtensions.cs	New extension methods providing backward-compatible overloads for Wait/WaitAsync with int timeout parameters
Source/Testably.Abstractions.Testing/Helpers/ExceptionFactory.cs	Added TimeSpan overload for TimeoutExpired exception factory method
Tests/Testably.Abstractions.Testing.Tests/NotificationTests.cs	Updated existing tests to use new API, marked obsolete filter tests with Obsolete attribute
Tests/Testably.Abstractions.Testing.Tests/NotificationTests.WaitAsync.cs	New test file with comprehensive WaitAsync test coverage including cancellation, disposal, and multiple wait scenarios
Tests/Testably.Abstractions.Testing.Tests/NotificationHandlerExtensionsTests.cs	Refactored tests to execute callbacks before Wait instead of using ExecuteWhileWaiting
Tests/Testably.Abstractions.Testing.Tests/FileSystem/ChangeHandlerTests.cs	Updated tests to use new Wait API pattern
Tests/Api/Testably.Abstractions.Api.Tests/Expected/*.txt	Updated API surface files for all target frameworks to reflect new public API

[github-actions]

Test Results

     52 files  ±  0       52 suites  ±0   36m 12s ⏱️ + 1m 28s
 63 908 tests + 24   60 293 ✅ + 24   3 615 💤 ±0  0 ❌ ±0 
132 002 runs  +115  117 458 ✅ +115  14 544 💤 ±0  0 ❌ ±0 

Results for commit 5993198. ± Comparison against base commit cbb6349.

♻️ This comment has been updated with latest results.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
86.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud