Fix SplitView stealing BackRequested when its pane is closed (#21740)

[gentledepp]

Fixes #21740

So while messing around with a mobile app I ran into this: on some pages the Android back button / back gesture just did… nothing 🤔

Took me a while, but the culprit turned out to be the SplitView. When it's in Overlay or CompactOverlay mode it hooks into TopLevel.BackRequested so it can close its pane on back. Makes sense - except it swallowed the event (e.Handled = true) even when the pane wasn't open at all. So the back navigation never got a chance to run.

The fix is tiny: if the pane is already closed, just bail out early and leave the event alone so it can bubble on to whoever actually wants to navigate back.

Behaviour now:

• pane open (overlay/compact overlay) → close the pane + mark handled ✅
• pane closed → don't touch it, Handled stays false → back navigation works again 🎉

Also threw in a couple of unit tests (both overlay modes) so we don't accidentally start stealing the event again when the pane is closed.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067385-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

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@alexander.marek,

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alexander.marek doesn't seem to be a GitHub user.
Please add the emails used in your commits to your Github account accordingly.

[gentledepp]

@cla-avalonia agree

[MrJul]

alexander.marek doesn't seem to be a GitHub user.

@gentledepp You need to change your commit settings to use an email address linked to your profile, or the CLA bot will complain each time.

[MrJul]

The changes make sense. LGTM!