Detected application hang message box does not respect unattended flag

In some cases we run our game package with `-unattended` to support some automated processes. However, occasionally a hang might happen and the process will detect this as part of the heartbeats. The main issue is that when this is detected it will open a message box to perform the user of this hang. This message box blocks the application from exiting, in our case we are trying to run the game in unattended and would like it to immediately continue and simply exit the game.

I’ve made a local change that replaces `FPlatformMisc::MessageBoxExt` to `FMessageDialog::Open` in `ThreadHeartbeat.cpp` and `RenderingThread.cpp`. `FMessageDialog` respects the unattended flag correctly.

I assume our setup is slightly unusual but I’d like to know if there are any specific reasons NOT to use `FMessageDialog` in these areas? And if not, why this isn’t used as standard for the message boxes (there are more use cases of `FPlatformMisc::MessageBoxExt` across the wider code base)

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Steps to Reproduce
Artificially force a hang in a game build with `-unattended` on the command line.

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Hi Ross,

To confirm, this issue is occurring on a Windows UE runtime correct?

Best regards

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Hi Ross,

I would guess that this may be an oversight as -unattended isn’t widely used against packaged builds. I’d be happy to run your changes by the development team if you wish to share.

Best regards.

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Hi Stéphane,

Yes, the issue happens on windows ue runtime (packaged game target).

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