[P2][HR][Gaps][Navigation][Test] Add regression tests for Region.Attached toggling across containers (bundle)

[kazo0]

Source

Sub-issue of unoplatform/private#926. Tracked from the HR + Navigation test matrix.

Status from matrix

Works — verified during the manual HR pass against the Chefs sample. (Bundle: 4 related rows.)

Scenario

Toggle uen:Region.Attached="True" on or off at runtime across multiple container types and assert the navigation hierarchy continues to function:

1. Outer/parent containers — Toggle uen:Region.Attached="True" on the outer Grid in MainPage.xaml (root layout). HR applies and navigation hierarchy keeps functioning.
2. RootGrid — Add/remove uen:Region.Attached="True" on RootGrid in MainPage.xaml. After HR, region navigation for the main content area continues to behave correctly.
3. TabsLayout — Toggle uen:Region.Attached="True" on TabsLayout in RecipeDetailsPage.xaml (the container for the details tabs). HR applies and the nested tab region still functions.
4. Inner TabBar — Toggle uen:Region.Attached="True" on the inner TabBar inside RecipeDetailsPage.xaml (Ingredients/Steps/Reviews/Nutrition tabs). HR applies and tab selection continues to drive navigation between tab regions.

Note: The related row "Add/Remove Region.Attached from TabBar (bottom MainPage)" was the only failing variation and is already tracked at #2971 (closed).

Goal

Add a Hot Reload regression test in the Uno.Extensions HR testharness covering each of the four Region.Attached toggle scenarios above.

Acceptance

• Regression test(s) added covering each of the four Region.Attached toggle variations
• Tests run as part of CI on the relevant platforms