Listview scrollbar with fallback focus support

[usmanvrtx]

Description

This PR enhances the TV scrollbar widget and ListView to handle the case when a ListView has no focusable TV content targets (e.g., only plain text items). In such cases, the scrollbar becomes the primary focus target and handles D-pad navigation directly, including blocking scroll-at-boundary from edge handoff.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

What Has Changed

• Added TVScrollbarFallbackFocusConfig class to provide focus coordinates when a ListView has no focusable content targets
• Added fallbackFocus parameter to TVScrollbarWidget to support fallback focus scenarios
• Added content focus probing logic in ListViewState to detect whether the list content contains any focusable TV targets via TVFocusRegistry and FocusManager
• Added ignoresTVFocus parameter to BoxWrapper to prevent double-focus wrapping when ListView uses internal scrollbar focus
• Updated TVTabButton to use TVFocusProviderScope when available for consistent focus offset handling
• Added _canScrollUp/_canScrollDown guards in TVScrollbarWidget to prevent scroll-at-boundary from being passed through to adjacent focusable elements
• Added lockHorizontalNavigation support for scrollbar in fallback mode to prevent horizontal focus escape
• Added comprehensive unit tests in tv_scrollbar_widget_test.dart covering fallback focus, scroll boundaries, and null content scenarios

How to Test

1. Run the new scrollbar tests: flutter test modules/ensemble/test/widget/tv_scrollbar_widget_test.dart
2. Verify that a ListView with only non-focusable items (e.g., plain Text widgets) correctly focuses the scrollbar and allows D-pad scrolling
3. Verify that a ListView with focusable items behaves as before (scrollbar focus via edge handoff)
4. Verify that scrollbar-at-boundary does not swallow navigation when in fallback mode

Screenshots / Videos

N/A

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors