feat(tv): scroll focused scrollbar into view

[usmanvrtx]

Description

This PR ensures a focused scrollbar is always revealed within its enclosing vertical scrollable on TV. Previously, when the scrollbar gained focus while positioned outside the visible viewport of a parent scrollable (e.g. a long ListView), it could remain hidden off-screen, making D-pad navigation confusing.

The change adds a reusable scroll-into-view helper that scrolls only the nearest vertical scrollable ancestor, so vertical positioning is fixed without jittering horizontal navigation.

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 modules/ensemble/lib/framework/tv/tv_focus_scroll.dart with:
  • findNearestVerticalScrollable(BuildContext) — finds the nearest vertical Scrollable ancestor
  • scrollWidgetIntoView(BuildContext, {verticalPadding, animationDurationMs, curve}) — scrolls only the nearest vertical scrollable so a widget's render box is fully visible, using padding from the viewport edges and clamping to valid scroll extents
  • Configurable constants: kTVScrollAnimationDurationMs, kTVVerticalScrollPadding, kTVScrollThreshold
• Updated TVScrollbarWidget in tv_scrollbar_widget.dart to call scrollWidgetIntoView(context) when the scrollbar gains focus, via a post-frame callback that re-checks _isFocused so stale callbacks are ignored
• This matches the same scroll-into-view rule already implemented for regular focusable widgets in box_wrapper.dart

How to Test

1. Run the relevant TV tests (if any): flutter test modules/ensemble/test/widget/tv_scrollbar_widget_test.dart
2. On a TV/emulator, focus a scrollbar whose parent is a long vertical scrollable — the scrollbar should be scrolled into view when focused
3. Move focus to an item that is fully visible — no scroll should occur (no jitter)
4. Navigate horizontally — vertical scroll position should remain unchanged
5. Verify regular focus navigation still works as before

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