[two_dimensional_scrollables] Fixes TreeView crash when empty or last node collapsed

[Piinks]

This PR provides a robust fix for a crash in TreeView and resolves a regression introduced by a previous partial fix in PR #9103.

Previous attempts to fix the "empty tree" crash (shrinking to 0 rows) by moving _updateScrollBounds to the end of layout introduced a subtle inconsistency. When a node collapse triggered a scroll correction, the visible row range was recalculated after the layout loop had finished. This resulted in the paint phase attempting to access children that were never built, causing a null dereference (as seen in the Expand then collapse with offscreen nodes (top) test).

• _updateVerticalScrollBounds is now called before the child layout loop. This ensures that any vertical scroll corrections (e.g., clamping when content shrinks) happen first, so the layout loop builds exactly the rows that will be visible.
• Accurate Extent Calculation: Updated _updateScrollBounds to calculate the vertical scroll extent using the total height of all rows in _rowMetrics (via _rowMetrics.last.trailingOffset). This provides a consistent maxScrollExtent and correctly handles the "empty tree" state.
• Correct Horizontal Bounds: Updated the horizontal extent calculation to use absolute content-relative positions, ensuring correct scroll bounds when scrolling horizontally.

Fixes flutter/flutter#164981

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[gemini-code-assist]

Code Review

This pull request updates the two_dimensional_scrollables package to version 0.5.2, addressing a crash in TreeView that occurred when the view shrunk to zero rows or when the last node was collapsed. The changes include safe-guarding the vertical extent calculation against empty row metrics and ensuring the layout loop only executes when rows are present. New test cases verify these fixes and ensure scroll bounds are updated correctly. Feedback indicates that the horizontal extent calculation should be updated to use content-relative positions to ensure correct scroll bounds during horizontal scrolling.

[Piinks]

Confirmed, this test was incorrect