Ensure correct exit code in case of cancellation and add OnExit phase for for IPushOnlyProtocol

[MarcoRossignoli]

This pull request introduces several changes to improve the handling of test session cancellation and ensure proper disposal of resources in the Microsoft.Testing.Platform project. The most important changes include initializing the exitCode with a default failure value, handling the cancellation token in various parts of the RunAsync method, and adding a new method to the IPushOnlyProtocol interface.

Improvements to test session cancellation handling:

• src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs: Initialized exitCode with ExitCodes.GenericFailure and added checks for testApplicationCancellationToken.IsCancellationRequested to set exitCode to ExitCodes.TestSessionAborted when necessary. [1] [2]

Resource disposal enhancements:

• src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs: Added a finally block to call PushOnlyProtocol.OnExitAsync() if PushOnlyProtocol is not null, ensuring proper cleanup.
• src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs: Updated DisposeServiceProviderAsync to continue if the service is an IPushOnlyProtocol, ensuring it is not disposed prematurely.

Interface and method additions:

• src/Platform/Microsoft.Testing.Platform/ServerMode/IPushOnlyProtocol.cs: Added the OnExitAsync method to the IPushOnlyProtocol interface.
• src/Platform/Microsoft.Testing.Platform/ServerMode/DotnetTest/DotnetTestConnection.cs: Implemented the OnExitAsync method to return a completed task.