Disabled RichTextBox renders BackColor in padding area but not in client area under NET11 VisualStylesMode

### .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

Under `VisualStylesMode = NET11`, a disabled `RichTextBox` renders the configured BackColor in the padding area, while the client area continues to use the disabled system background color.
As a result, different background colors are displayed within the same control, creating a visually inconsistent disabled appearance.

**No padding value set, only BackColor set:**

**Set padding to 20, and set BackColor:**

### Steps to reproduce

1. Create a WinForms application targeting .NET 11.
2. Add a `RichTextBox` to the form
3. Set `Enable` to false, set `Padding` and `BackColor`
4. Set `VisualStylesMode` to `NET11`
5. Run the application to observe

Start by reproducing the issue in a .NET 11 WinForms application with a disabled RichTextBox, Padding, BackColor, and VisualStylesMode set to NET11. Trace the RichTextBox rendering path and compare the padding and client-area backgrounds; done means both areas render consistently in the disabled state.

Written by the indexing model from the issue text.