WinUI 3 Slider thumb is clipped by bounding box on differing DPI displays.

### Describe the bug

When moving a window between displays of differing DPI scaling (e.g. 125% and 150%), the slider thumb is clipped by it's bounding box, giving the appearance of a rounded rectangle. This appears to be cosmetic only.

### Why is this important?

If using the provided thumb or custom thumb, the thumb can become cropped leading to an uneven/inconsistent experience across the application.

### Steps to reproduce the bug

Requires a system with 2 displays.
1. Set first display to 125% scaling at 1920x1080 resolution
2. Set second display to 150% scaling at 1920x1080 resolution
3. Bring up the slider control in WinUI 3 Gallery application
4. Move the application between the monitors. (Sometimes requires changing resolutions of the monitors to reproduce, also display edge snapping for applications seemed to increase frequency of occurrence).

### Actual behavior

The slider's thumb appearance is clipped by the bounding box of the control.

### Expected behavior

The slider's appearance should remain consistent, i.e. no clipping.

### Screenshots

### NuGet package version

Microsoft.WindowsAppSDK.WinUI 1.8.260204000

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start by reproducing the issue in the WinUI 3 Gallery with two displays set to 125% and 150% scaling, then trace the Slider thumb's layout and bounding behavior during monitor changes. Done means the provided and custom thumbs remain fully visible and consistently shaped when moving the window between differing-DPI displays.

Written by the indexing model from the issue text.