[Testing] Fix net11 CI failure in Picker UI test (Issue34971)

[NafeelaNazhir]

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Description of Change

On iOS 26, the picker dialog's action button label changed from "Done" to "selected", which broke the Issue34971 UI test that hard-coded the "Done" accessibility id when dismissing the Picker dialog. This PR updates the test to dynamically resolve the correct button label based on the iOS version under test, using the existing HelperExtensions.IsIOS26OrHigher helper.

Changes:

• Added a doneButton computed property in Issue34971 that returns "selected" on iOS 26+ and "Done" otherwise (via App is AppiumIOSApp iosApp && HelperExtensions.IsIOS26OrHigher(iosApp)).
• Updated App.WaitForElement/App.Tap calls in CloseOpenedPicker to use this computed value instead of the hard-coded "Done" string.

Issue Fixed
Fixes #36857

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.sh | bash -s -- 36873

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iex "& { $(irm https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.ps1) } 36873"

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Pull request overview

Updates the Issue34971 Picker UI test to handle an iOS behavior change where the picker confirmation button’s accessibility label differs on iOS 26+, preventing iOS UI test timeouts when dismissing the picker dialog.

Changes:

• Added a computed doneButton selector that returns "selected" on iOS 26+ and "Done" otherwise.
• Updated CloseOpenedPicker() to use doneButton for iOS/MacCatalyst picker dismissal instead of a hard-coded "Done".