Fix SplitPane RangeError when child count changes between rebuilds (#9822)

* Fix SplitPane RangeError when child count changes between rebuilds

Fixes #9648.

SplitPane cached its fractions list in initState only. When the parent
rebuilt the widget with a different number of children (for example,
toggling a panel via a collection-if), fractions.length stayed at the
old value while widget.minSizes and widget.children shrank, causing
minSizeForIndex to read past the end of widget.minSizes and throw
'RangeError (index): Index out of range: index should be less than 2: 2'
from the layout pass.

This adds didUpdateWidget to _SplitPaneState. When the child count
changes, fractions is reset to List.of(widget.initialFractions) so it
stays in sync with the new children and minSizes. The existing
constructor assertion already guarantees children.length matches
initialFractions.length.

Bumps devtools_app_shared to 0.5.2 with a CHANGELOG entry, and adds
regression tests that pump a 3-child SplitPane and then a 2-child
SplitPane (and vice versa) and assert no exception is thrown.

* Add release note entry for SplitPane RangeError fix (#9822)

* Address review: bump devtools_app_shared CHANGELOG to 0.5.2-wip

* Address review: bump devtools_app_shared pubspec to 0.5.2-wip