Fix CommandBar secondary visibility during overflow open

[NathanDrake2406]

What does the pull request do?

Fixes a CommandBar overflow crash when a secondary command changes IsVisible while the overflow popup is realizing its items.

The scenario is reported in #21694: a CommandBarButton in CommandBar.SecondaryCommands can update IsVisible from a binding as it is added to the popup's ItemsControl, which re-enters the command bar overflow rebuild.

What is the current behavior?

Opening the overflow popup can throw:

InvalidOperationException: The control CommandBarButton already has a visual parent StackPanel while trying to add it as a child of StackPanel.

The re-entrant visibility update clears and rebuilds OverflowItems while PanelContainerGenerator.InsertContainer is still inserting the same control into the popup panel.

What is the updated/expected behavior with this PR?

Opening the overflow popup no longer mutates OverflowItems during the unsafe synchronous item-realization window. A visibility-triggered overflow rebuild is deferred until after Popup.IsOpen = true returns, then applied immediately so existing synchronous overflow state behavior is preserved outside that opening path.

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

CommandBar now routes dynamic overflow rebuild requests through a small guard. While the overflow popup is synchronously opening, rebuild requests are marked as deferred. After the popup finishes opening, a single deferred rebuild is applied.

A regression test covers a secondary command that changes IsVisible as it becomes parented by the overflow presenter, reproducing the same visual-parent collision path without depending on a particular binding engine timing detail.

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

None.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #21694

Verification

• dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj -- --filter-method "*Open_DoesNotThrow_WhenSecondaryCommandVisibilityChangesDuringOverflowRealization"
• dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj -- --filter-class "Avalonia.Controls.UnitTests.CommandBar*"
• dotnet build src/Avalonia.Controls/Avalonia.Controls.csproj -p:AvsSkipBuildingLegacyTargetFrameworks=True
• dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067390-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!