[two_dimensional_scrollables] Fixes TreeView crash when empty or last node collapsed (#11622)

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 https://github.com/flutter/flutter/issues/164981

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