Add Windows app model acceptance coverage

[Amaury Levé (Evangelink)]

MSTest ships specialized assets for classic UWP, Modern UWP, and WinUI, but only unpackaged WinUI had real end-to-end acceptance coverage. This adds executable coverage for the missing packaged application models so asset-selection, activation, UI dispatch, and cleanup regressions are caught before release.

• Build and execute real classic UWP and Modern UWP packages through VSTest .build.appxrecipe files.
• Register and activate a real full-trust packaged WinUI app through MTP, including controller connect-back and UI-thread execution.
• Verify the packed MSTest NuGets contain and resolve the expected UWP and WinUI assets.
• Remove and verify package registrations after every run, with a pipeline leak check as a final guard.
• Run activation tests only in a dedicated preflight-gated Windows job; ordinary and official test runs skip the environment-specific category.

Validation covered package creation, all three application-model activation paths, package asset contracts, disabled-environment gating, and package-registration cleanup.

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