Honor STAThread entry point for MSTest execution

[Copilot]

MSTest only honored explicit <ExecutionThreadApartmentState> configuration, so tests launched from an [STAThread] Main could fall back to MTA thread-pool execution after async continuations. This made the no-runsettings STA acceptance scenario timing-dependent and flaky in Debug.

• Apartment state resolution
  • Added shared effective apartment-state logic:
    • explicit runsettings/config value wins
    • otherwise [STAThread] on the entry assembly entry point implies STA on Windows
    • non-Windows behavior remains unchanged

ApartmentState? requestedApartmentState =
    MSTestSettings.GetEffectiveExecutionApartmentState();

• Execution flow

  • Updated both MSTest execution paths to use the effective apartment state:
    • MSTestExecutor.RunTestsFromRightContextAsync
    • TestExecutionManager.DefaultFactoryAsync

• Coverage

  • Re-enabled the no-runsettings STA acceptance test.
  • Added focused unit coverage for explicit MTA precedence, implicit Windows STA, non-Windows behavior, and non-STA entry points.

[Youssef1313]

We explicitly don't await the test when running under STA to preserve the STA thread:

testfx/src/Adapter/MSTestAdapter.PlatformServices/Execution/TestExecutionManager.cs

Lines 500 to 511 in 52aa5cd

	if (usesAppDomains || Thread.CurrentThread.GetApartmentState() == ApartmentState.STA)
	{
	#pragma warning disable VSTHRD103 // Call async methods when in an async method - We cannot do right now because we are crossing app domains.
	// TODO: When app domains support is dropped, we can finally always be calling the async version.
	// In addition to app domains, if we are STA thread (e.g, because runsettings setting ExecutionApartmentState to STA), we want to preserve that.
	// If we await, we could end up in a thread pool thread, which is not what we want.
	// Alternatively, if we want to use RunSingleTestAsync for the case of STA, we should have:
	// 1. A custom single threaded synchronization context that keeps us in STA.
	// 2. Use ConfigureAwait(true).
	unitTestResult = testRunner.RunSingleTest(unitTestElement, testContextProperties, remotingMessageLogger);
	#pragma warning restore VSTHRD103 // Call async methods when in an async method
	}

I'm not sure what exactly is breaking here and at which point do you lose the STA thread.

[Youssef1313]

Okay I think I can see what's going on here.

I think the test is written incorrectly and should be changed to use the MSTest configuration.

We never well supported [STAThread] explicitly on entrypoints and it looks unnecessary to do so, and no user should ever be using the GetAwaiter().GetResult() stuff that exist in this test. The test case is a bad practice that shouldn't be encouraged.

[Copilot]

Pull request overview

This PR improves MSTest execution on Windows by honoring an [STAThread] entry-point when no explicit <ExecutionThreadApartmentState> is configured, reducing timing-dependent STA/MTA behavior during MSTest runner execution.

Changes:

• Added centralized “effective apartment state” resolution that prioritizes runsettings, otherwise infers STA on Windows from [STAThread] on the entry point.
• Updated both MSTest execution paths (VSTest adapter and platform services execution manager) to use the effective apartment state.
• Re-enabled and adjusted STA acceptance coverage, and added targeted unit tests for precedence and platform behavior.

Show a summary per file

File	Description
src/Adapter/MSTestAdapter.PlatformServices/MSTestSettings.cs	Adds shared effective apartment-state resolution (runsettings override, otherwise infer STA on Windows from entry point).
src/Adapter/MSTestAdapter.PlatformServices/Execution/TestExecutionManager.cs	Uses the effective apartment state when deciding whether to run work on an STA thread.
src/Adapter/MSTest.TestAdapter/VSTestAdapter/MSTestExecutor.cs	Uses the effective apartment state when selecting the “right context” for execution.
test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/MSTestSettingsTests.cs	Adds unit coverage for precedence (configured wins), Windows STA inference, non-Windows behavior, and non-STA entry points.
test/IntegrationTests/MSTest.Acceptance.IntegrationTests/STAThreadingTests.cs	Re-enables and strengthens STA entry-point acceptance scenario (no-runsettings case) and adjusts ExecuteAsync invocation formatting/inputs.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 0