feat(itemspresenter): add support for footer and header, as well as irregular snap points

[ramezgerges]

GitHub Issue (If applicable): closes #10745, closes #13248

PR Type

What kind of change does this PR introduce?

• Feature

What is the current behavior?

What is the new behavior?

Copilot Summary

🤖 Generated by Copilot at 10f86b8

This pull request improves the StackPanel and ItemsPresenter controls in Uno Platform by fixing snap point calculation, adding header and footer support, and increasing test coverage. It also simplifies the use of the WindowHelper class in the StackPanel unit tests and refactors some common code.

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

Internal Issue (If applicable):

[jeromelaban]

Small changes remaining, but looks good!

[jeromelaban]

Very nice! There are a few tests like

uno/src/SamplesApp/SamplesApp.UITests/Windows_UI_Xaml_Controls/ListViewTests/UnoSamples_Tests.ListView.cs

Line 90 in 85ad12d

[ActivePlatforms(Platform.Android)] // Fails on iOS - https://github.com/unoplatform/uno/issues/1955

that could benefit from being enabled for wasm. Could you take a look?

[jeromelaban]

@ramezgerges There's one test that fails:

https://dev.azure.com/uno-platform/Uno%20Platform/_TestManagement/Runs?runId=2508900&_a=resultSummary&resultId=100001

ListView_ExpandableItemLarge_ExpandHeader_Validation

ImageAssert.AreEqual @ line 326
pixelTolerance: Color Cumulative tolerance of 5 | Location PerRange tolerance of 0,0 pixels.
expected: Initial State (ListView_ExpandableItemLarge_ExpandHeader_Validation_Initial_State.png 1024x768) in {X=0,Y=0,Width=1024,Height=768}
actual  : Collapsed State (ListView_ExpandableItemLarge_ExpandHeader_Validation_Collapsed_State.png 1024x768) in {X=0,Y=0,Width=1024,Height=768}
====================
Initial State:
132,68: expected: [132,68] FF333333 | actual: [132,68] FF000000
	a tolerance of 153 [Cumulative] would be required for this test to pass.
	Current: Color Cumulative tolerance of 5 | Location PerRange tolerance of 0,0 pixels.


With expectedRect:
{X=0,Y=0,Width=2147483647,Height=2147483647}
With actualRect:
{X=0,Y=0,Width=2147483647,Height=2147483647}
With expectedToActualScale:
1