VirtualizedListCellRenderer: hoist zIndex from renderItem element

[siddeshgawande]

Summary

Fixes an issue where zIndex applied to elements returned by renderItem was not respected by VirtualizedListCellRenderer, causing incorrect stacking order in FlatList/VirtualizedList items.

Changes

• Add extractZIndexFromListItemElement to retrieve zIndex from the outermost element returned by renderItem.
• Support Fragment children by extracting the maximum zIndex among Fragment descendants.
• Hoist extracted zIndex onto the cell wrapper style so sibling cells render in the correct stacking order.
• Preserve existing inversion and layout styles using StyleSheet.compose.
• Add unit tests covering:
  • direct root element zIndex
  • style array flattening
  • Fragment children
  • absence of zIndex
  • custom CellRendererComponent support

Fixes #18616

Changelog:

[General] [Fixed] - Preserve zIndex from renderItem elements in VirtualizedListCellRenderer

Test Plan

• Added unit tests for zIndex extraction behavior.
• Verified zIndex is correctly applied for:
  • direct root elements
  • Fragment children
  • style arrays
  • custom CellRendererComponent usage
• Ran VirtualizedList test suite successfully.

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