Fix unit test host crash on iOS 27 by adopting the UIScene life cycle

[crazytonyli]

iOS 27 removed the legacy window life cycle, so the unit test host's plain UIWindow() crashed at launch with "UIScene life cycle is required for apps built with this SDK" before the test runner could connect. This blocked every app-hosted (WordPressTest) suite on iOS 27 while the same binary ran on iOS 26.5.

TestingAppDelegate now opts into the scene life cycle the same way WordPressAppDelegate does, vending a minimal TestingSceneDelegate that builds the host window in scene(_:willConnectTo:). The window shows a SwiftUI "Running unit tests" placeholder. Verified the previously crashing suites pass on both iOS 27 and iOS 26.5.

[wpmobilebot]

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	33922
	Version	PR #25930
	Bundle ID	org.wordpress.alpha
	Commit	ff879ae
	Installation URL	4rg81t3krg58g

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33922
	Version	PR #25930
	Bundle ID	com.jetpack.alpha
	Commit	ff879ae
	Installation URL	5e9vkbk1e9v50

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[jkmassel]

nit:

WordPress-iOS/Modules/Sources/WordPressSharedUI/UIApplication+MainWindow.swift

Lines 5 to 11 in 36a39a2

	// The delegate-window fallback covers the moments when no scene key window
	// exists: early in scene connection (before makeKeyAndVisible) and in the unit
	// test host, which never connects a window scene.
	connectedScenes
	.compactMap { ($0 as? UIWindowScene)?.keyWindow }
	.first
	?? (delegate?.window).flatMap { $0 }

mentions "never connects a window scene", which IINM this PR makes incorrect.

[crazytonyli]

@jkmassel Good catch! Updated in the latest commit.