Implement list scrolling to keep selected item visible in long test lists

[Copilot]

When navigating test lists longer than the screen height, the selected item could scroll off-screen, making it difficult for users to track their current position. This PR implements viewport management for the list panel to ensure the selected item always remains visible.

What's Changed

Viewport Management

• Added ListScrollPosition and ListHeight parameters to Get-ListPanel
• Implemented scrolling logic similar to the existing preview panel
• Only renders visible items within the viewport height
• Displays scroll indicators ("..." and "...more") when content extends beyond viewport

Smart Navigation

• Arrow keys: Auto-scroll to keep selected item visible when moving up/down
• Page Up/Down: Jump 10 items with optimal scroll positioning
• Home/End: Reset to top/bottom with appropriate scroll state
• Level navigation: Clean scroll reset when entering/exiting directories

Example Behavior

Before this change:

│ Test15  (selected, but off-screen)
│ Test16
│ Test17
│ Test18
│ Test19
│ Test20

After this change:

│ ...
│ Test13
│ Test14
│ ▶ Test15  (selected, always visible)
│ Test16
│ ...more

Technical Details

The implementation adds a separate $listScrollPosition variable to track list viewport state independently from the preview panel scroll position. Navigation logic calculates appropriate scroll offsets to maintain selected item visibility while providing visual feedback about content availability above and below the current viewport.

All changes maintain backward compatibility and follow existing code patterns. Zero breaking changes to current functionality.

Fixes #11.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[github-actions]

Test Results

  3 files   51 suites   3s ⏱️
 57 tests  41 ✅ 2 💤 14 ❌
168 runs  118 ✅ 6 💤 44 ❌

For more details on these failures, see this check.

Results for commit 18ab4f4.

♻️ This comment has been updated with latest results.