Apply A2UI CardBackground theme override

[Copilot]

A2UITheme parsed colors.card, but CardRenderer ignored it and always used the default Fluent card background. This made the documented card background theme override a no-op for agent-provided surfaces.

• Renderer
  • Applies ctx.Theme.CardBackground to the rendered card Border.Background when present.
  • Preserves the existing Fluent resource fallback when no override is provided.

if (ctx.Theme.CardBackground is { } cardBackground)
    border.Background = new SolidColorBrush(cardBackground);
else if (Application.Current.Resources.TryGetValue("CardBackgroundFillColorDefaultBrush", out var bg) && bg is Brush bgBrush)
    border.Background = bgBrush;

• Coverage
  • Adds a focused A2UI UI test for styles.colors.card rendering as the card border background.

[shanselman]

Closing this automation implementation as stale/superseded after the post-refactor triage. The underlying signal is preserved in #343 and the tracker update in #372: CardBackground is tracked in #372 and has duplicate/stale automation coverage.