fix(controls): fix radio MenuItem grouping in MenuFlyout

[zrt2399]

What does the pull request do?

fix #21355

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

MenuFlyout radio MenuItems were not participating in radio group updates
the same way as ContextMenu items.

When MenuItems are hosted in a MenuFlyout, they may rely on the visual tree
to find their owning MenuBase/interaction handler after realization. As a
result, radio items could stay checked instead of unchecking other items in
the same group.

This change:

• adds a visual-tree fallback when resolving MenuItem interaction handlers
• registers radio MenuItems when they attach to the visual tree
• guards RadioButtonGroupManager against duplicate registrations
• adds a regression test covering MenuFlyout radio group behavior

This fixes the case where grouped radio MenuItems inside MenuFlyout do not
behave mutually exclusively, while the equivalent ContextMenu setup works.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

fix #21355

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065347-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!