Fixes #4920. Re-enter RunLoop when End is cancelled by IsRunningChanging

[YourRobotOverlord]

Fixes #4920.

Problem

See the linked issue. When OnIsRunningChanging (or an IsRunningChanging event handler) vetoes a stop, ApplicationImpl.Run() returned early leaving the runnable with IsRunning=true, StopRequested=true, and no event loop active — a freeze or crash.

Fix

In ApplicationImpl.Run(), wrap the RunLoop / End block in a while(true) loop and only break when End() succeeds (i.e. runnable.IsRunning == false). When End is cancelled, reset StopRequested=false and re-enter RunLoop:

// BEFORE
try
{
    RunLoop(runnable, errorHandler);
}
finally
{
    End(token);
}

// AFTER
while (true)
{
    try
    {
        RunLoop(runnable, errorHandler);
    }
    finally
    {
        End(token);
    }

    if (!runnable.IsRunning)
        break;

    // End was vetoed — reset StopRequested so RunLoop can re-enter correctly
    runnable.StopRequested = false;
}

Why This Is Safe

Scenario	Before	After
Normal stop	✅ works	✅ same — IsRunning becomes false after End, loop breaks
Exception in RunLoop	✅ End runs in finally, exception propagates	✅ same
Stop vetoed by IsRunningChanging	❌ Run() returns early; runnable dead	✅ RunLoop re-entered; runnable stays interactive

Changes

File	Change
Terminal.Gui/App/ApplicationImpl.Run.cs	Wrap try/finally in while(true) loop
Tests/UnitTestsParallelizable/Application/ApplicationImplTests.cs	Regression test Run_WhenStopCancelledByIsRunningChanging_RunLoopResumes
Examples/UICatalog/Scenarios/IsRunningChangingCancelDemo.cs	New UICatalog scenario demonstrating the OnIsRunningChanging veto pattern

[tig]

Lovely! See my request.