[ChipGroup] default selection enforced by ChipSelectionMode=Single, no longer work due to ItemsPanelRoot's new resolution timing

[Xiaoy312]

Current behavior

^ title

Expected behavior

Default selection should be enforced with ChipSelectionMode=Single.

How to reproduce it (as minimally and precisely as possible)

• bump toolkit to an Uno.WinUI that contains the TemplatedParent rework (which impacted ItemsPanelRoot resolution timing)
• launc the sample app
• run this test: VariousMode_TapSelection
• notices the ChipSelectionMode.Single set of tests fails

Package Version(s): [email protected]
Affected platform(s): all
IDE: don't matter
Relevant plugins: n/a

Anything else we need to know?

Somewhat related to #1281, but that one can be worked around by accessing the containers via ItemsControl::Items instead, since we have xaml-defined items, while here we are dealing with ItemsSource'd items...