VisualElementFocusRing retains strong reference to removed VisualElements when focus ring cache is not invalidated on detach

How to reproduce:

1. Open the attached "IN-152599" project
2. Go to Window → General -> Test Runner
3. In the Test Runner, select the “PlayMode” tab
4. At the bottom, press “Run All”
5. Observe the results in the Test Runner window

Actual result: A message "BUG REPRODUCED: VisualElementFocusRing still holds a strong reference to buttonB after RemoveFromHierarchy()..." is generated after the test fails
Expected result: The test passes with no error messages

Reproducible with: 6000.0.0f1, 6000.0.83f1, 6000.3.23f1, 6000.6.0f1, 6000.7.0a6

Reproducible on: Windows 11, macOS 26.6.2 (by user)
Not reproducible on: No other environments tested