Fixes #5579. SynchronizationContext is not correctly implemented in v2

[BDisp]

Fixes

• Fixes SynchronizationContext is not correctly implemented in v2 #5579

Proposed Changes/Todos

• The SynchronizationContext is set on the Init method and also on the Begin method. Without the fix the unit tests will be fail.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[Copilot]

Pull request overview

Addresses #5579 by restoring a Terminal.Gui main-loop-backed SynchronizationContext so async/await continuations resume on the app’s main thread in v2.

Changes:

• Reintroduces MainLoopSyncContext and sets it during ApplicationImpl.Init and session Begin.
• Adds unit tests asserting SynchronizationContext.Post runs on the app’s main thread and is instance-specific.
• Updates MainLoopCoordinator to avoid capturing synchronization context during startup waits.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Tests/UnitTestsParallelizable/Application/ApplicationImplTests.cs	Adds tests validating SynchronizationContext posts back to the app main thread and is per-app-instance.
Terminal.Gui/App/MainLoop/MainLoopSyncContext.cs	Implements an app-instance-backed SynchronizationContext that marshals work to the app via Invoke.
Terminal.Gui/App/MainLoop/MainLoopCoordinator.cs	Uses ConfigureAwait (false) while awaiting startup tasks to avoid context capture.
Terminal.Gui/App/ApplicationImpl.Run.cs	Ensures the app’s main-loop sync context is set when a session begins.
Terminal.Gui/App/ApplicationImpl.Lifecycle.cs	Creates and installs the app’s MainLoopSyncContext during Init.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.