DropDownList: dropped (expanded) list should use a background attribute distinct from its SuperView

[tig]

Describe the bug

When a DropDownList is dropped (expanded), the open list popup does not use a background attribute distinct from its SuperView's. Because the dropped list inherits/blends with the parent's scheme, the popup is hard to distinguish from the content behind/around it — it should "float" with its own background (e.g. the Menu/popover scheme, or at least a visibly different attribute) so the user can tell the list is open and where its bounds are.

This is especially visible when the parent View/Window uses a scheme whose Normal background matches the list's (e.g. an app that sets its top-level window to the Accent/Base scheme): the expanded DropDownList is nearly invisible against the SuperView.

I'm fairly sure I fixed this at some point — filing because it appears to be back (possible regression). Flagging so we can add a regression guard once fixed.

To Reproduce

Steps to reproduce the behavior:

1. Place a DropDownList inside a Window/View and give that container a scheme (or rely on the default) whose Normal background matches the list's.
2. Expand the DropDownList.
3. Expected behavior: the dropped list renders with its own, distinct background attribute (clearly separated from the SuperView — like a popover/menu), so the open list is visually obvious.
4. Actual behavior: the dropped list shares the SuperView's background attribute, so the open list blends into the surrounding content and its bounds are hard to see.

Environment

• Terminal.Gui: 2.4.7
• OS: macOS (Darwin 25.5.0)
• .NET: 10
• Terminal/host: seen under the agg/sixel render path while driving a TUI, but it's a scheme/attribute issue independent of the host.

Notes / suggested fix

The dropped portion of DropDownList should resolve its own scheme (e.g. a Menu/popover scheme) rather than inheriting SuperView.GetScheme(), so its Normal attribute differs from the SuperView by construction. A golden/attribute test that asserts the expanded list's background attribute != the SuperView's would prevent regressions.