Use AddRange switch to preserve equal-node order

[Rick-Xu-WTG]

What

Use the existing WinForms AppContext switch to restore the legacy AddRange order for the demo scenario:

AppContext.SetSwitch("System.Windows.Forms.TreeNodeCollectionAddRangeRespectsSortOrder", false);

The demo now sets the switch at startup before the first sorted AddRange call.

Why

The reversal happens only when the Pallet nodes compare equal. AddRange iterates nodes backwards, and AddSorted inserts a new equal item after existing equal items. With equal comparisons, that turns the input order into a full reverse.

The switch from dotnet#11423 already controls whether AddRange sets FixedIndex, but Sorted=true still entered AddSorted before the fixed-index path could matter. This PR keeps the source change minimal: while AddRange is using FixedIndex, skip the sorted insertion/sort path so the switch can actually restore the legacy order.

Verification

==== --treeview-addrange-report ====
switch OFF ->
Input:  Pallet 1,Pallet 2,Pallet 3,Pallet 4,Pallet 5
Output: Pallet 1,Pallet 2,Pallet 3,Pallet 4,Pallet 5
Reversed: False

==== --treeview-addrange-report --switch:false ====
switch OFF ->
Input:  Pallet 1,Pallet 2,Pallet 3,Pallet 4,Pallet 5
Output: Pallet 1,Pallet 2,Pallet 3,Pallet 4,Pallet 5
Reversed: False

==== --treeview-addrange-report --switch:true ====
switch ON ->
Input:  Pallet 1,Pallet 2,Pallet 3,Pallet 4,Pallet 5
Output: Pallet 5,Pallet 4,Pallet 3,Pallet 2,Pallet 1
Reversed: True

Builds:

.\.dotnet\dotnet.exe build src\test\integration\WinformsControlsTest\WinformsControlsTest.csproj -c Debug --no-restore
.\.dotnet\dotnet.exe build src\test\unit\System.Windows.Forms\System.Windows.Forms.Tests.csproj -c Debug --no-restore