Page Up/Down in ListView goes to top or bottom of list

### Describe the bug

When a `ListView` is expanded so that all items are visible in the parent control, _Page Up_ and _Page Down_ have the same effect as _Home_ and _End_, navigating to the top and bottom of the list, respectively. That's because the paging in `ListView` is dependent on the height of the control instead of the height of the actual viewable are of the list view.

### Why is this important?

This was reported as an accessibility issue in our app.

### Steps to reproduce the bug

This is visible in the Windows Settings app: Go to the _Installed App_ page. With focus on an app, press _Page Down_, _Page Up_.

### Actual behavior

Focus moves to the bottom or top of the list.

### Expected behavior

Focus should move in increments based on what's visible.

### NuGet package version

Verified with the latest WinUI 3 Gallery 2.9.3.0

### Windows version

Windows 11 (25H2): Build 26200

Start by reproducing the issue in the Windows Settings Installed Apps page or WinUI 3 Gallery 2.9.3.0 with focus on an app in a fully visible ListView. Trace how ListView calculates Page Up and Page Down movement relative to the visible area, then verify that paging moves by visible increments rather than directly to the top or bottom.

Written by the indexing model from the issue text.