UI layout misalignment in AI Assistant mode dropdown

Steps to reproduce:

1. Create a Unity project
2. Select "Light" Unity theme
3. Click the "AI" button
4. Click "Agree and install Unity AI"
5. Wait until the installation is complete
6. Click on the "Mode Dropdown" and verify that it displays data correctly
7. Open the Package Manager, select "In Project"
8. Enter "Assist" into the search field
9. Verify that the "Assistant" package is found
10. Select the "Version History" tab and click "See other versions"
11. Select  2.1.0-pre.1 and click "Update"
12. Return to the "Assistant" window and verify that the "Mode Dropdown" can not be selected
13. Return to the Package Manager window and install 2.5.0-pre.2 package version
14. Return to the "Assistant" window and verify that the "Mode Dropdown" one more time

Actual results: The menu items ("Agent", "Plan", "Ask") appear significantly offset from the button. They render as plain text without a background container and overlap the "Build with Unity" label, making the UI difficult to read and interact with.

Expected results: The dropdown menu should appear directly above or below the button, properly contained within a menu container, and should not overlap background text or appear "floating" in the middle of the window

Reproducible with versions:  2.5.0-pre.2

Tested on (OS): macOS - Tahoe 26.4

Notes: The console displays this error

Missing types referenced from component VisualTreeAsset on game object AssistantTextField:	Unity.AI.Assistant.UI.Editor.Scripts.Components.PopupSelector/UxmlSerializedData, Unity.AI.Assistant.UI.Editor (2 objects)