TextBox's clear button not tablable and not screen reader

### Describe the bug

When input in the textbox the clear button appears, clicking on the button does clear the text. But the clear button is not tabable.

Even when the clear button can be referenced via styling, making the button tabable removes the visibility support and the onclick functionality.

### Steps to reproduce the bug

1. Use a textbox (or checkout WinUi Gallery's Textbox sample)
2. Input text
3. Clear button appears

### Expected behavior

When input in the textbox the clear button appears, clicking on the button does clear the text. But the clear button is not tabable.

Even when the clear button can be referenced via styling, making the button tabable removes the visibility support and the onclick functionality.

### Screenshots

https://github.com/user-attachments/assets/d1702435-a690-4efc-855d-67df18b98c30

### NuGet package version

None

### Windows version

_No response_

### Additional context

Original issue: https://github.com/microsoft/PowerToys/issues/41089

Start with the WinUI Gallery TextBox sample and reproduce the issue using keyboard navigation and a screen reader. Then trace the TextBox clear-button behavior and its styling or control entry points. Done means the visible clear button is keyboard reachable and announced by assistive technology while preserving its visibility and click behavior.

Written by the indexing model from the issue text.