UI Builder adds non-removable built-in class selectors when adding a custom selector via “Add Selector...”

Steps to reproduce:

1. Create a new project using Universal 3D template
2. Go to Assets > Create > UI Toolkit > UI Document (UXML)
3. Open the created UI Document in UI Builder
4. Add any element to Hierarchy, e.g. "Toggle"
5. Right-click on it and select "Add Selector..."
6. In the selector field, type in any name (e.g. ".test") and click enter
7. Click on the previously created element ("Toggle") and observe its Style Class List in Inspector

Actual results: "Toggle" elements has ".unity-toggle" selector in the Style Class List and it is not removable

Expected results: "Toggle" elements has ".test" selector in the Style Class List and it is removable

Reproducible with versions: 6000.0.66f1, 6000.3.8f1, 6000.4.0b8, 6000.5.0a6

Tested on (OS): Ubuntu 24.04, Windows 11

Note: 

• Reproduces across multiple UI Toolkit controls, not limited to Toggle
• The built-in class appears to override or replace the custom selector added via “Add Selector...” and cannot be removed from the Style Class List