RichTextBox is not fully borderless with BorderStyle=None under VisualStylesMode.NET11

### .NET version

.NET 11 RC2 build: 11.0.100-rc.2.26455.110

### Did it work in .NET Framework?

Not tested/verified

### Did it work in any of the earlier releases of .NET Core or .NET 5+?

Occurs after New APIs in .NET11

### Issue description

When `RichTextBox.BorderStyle` is set to `None `and `VisualStylesMode `is configured as `NET11`, visible border strokes are still rendered along the top and left edges of the control.
This behavior differs from the `Classic `visual style implementation, where no border is rendered when `BorderStyle=None`.

### Steps to reproduce

1. Create a RichTextBox.
2. Set BorderStyle = None.
3. Set VisualStylesMode = NET11.
4. Run the application and observe the control.

**Actual Result**
Thin border lines remain visible on the top and left sides of the RichTextBox.
**Expected Result**
No border should be rendered on any side of the RichTextBox when BorderStyle=None, matching the behavior of Classic mode.

Start by reproducing the issue with a RichTextBox configured with BorderStyle=None and VisualStylesMode=NET11, then compare its border rendering with Classic mode. Trace the RichTextBox and visual-style border implementation; done means no border strokes appear on any side in NET11 mode, matching the stated expectation.

Written by the indexing model from the issue text.