Add acceptance test for hang dump triggering after test session finish

[Copilot]

Adds test coverage for the hang dump fix (#7314) that ensures hang dumps trigger even when foreground threads continue running after test execution completes.

Test Implementation

• New test HangDump_WithBackgroundThreadAfterSessionFinish_CreateDump spawns a foreground thread that hangs after tests finish
• Test asset modified to spawn thread when SPAWN_BACKGROUND_THREAD environment variable is set
• Verifies dump file creation after 8-second timeout despite session completion

The test validates that removing session end coordination (previous fix) allows hang dump monitoring to continue post-session:

// In test asset's ExecuteRequestAsync
if (Environment.GetEnvironmentVariable("SPAWN_BACKGROUND_THREAD") == "true")
{
    Thread backgroundThread = new Thread(() => Thread.Sleep(600000));
    backgroundThread.IsBackground = false; // Foreground thread prevents process exit
    backgroundThread.Start();
}
context.Complete(); // Session ends but process hangs

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