Make experimental samples version-driven

[shisan (qiutongMS)]

Description

Consolidates the stable and experimental Windows AI Foundry WinUI scenarios into one source tree. The Windows App SDK version selected in Samples/Directory.Packages.props now determines which scenarios are built.

This change:

• Introduces WindowsAppSDKVersion as the canonical Windows App SDK version updated by UpdateVersions.ps1.
• Derives whether the selected package is experimental from that version.
• Extracts code shared by stable and experimental scenarios into a separate WindowsAISample.Shared project.
• Builds the Video Scaler and Image Foreground Extractor extensions only when an experimental Windows App SDK package is selected.
• Removes stale extension artifacts when switching from an experimental build back to a stable build.
• Documents the stable and experimental build behavior in the Windows AI Foundry sample README.

This keeps the stable sample unchanged while allowing experimental scenarios to live in the same branch without requiring separate source trees.

Validation

Validated with both package channels:

• Stable (2.4.0): the Shell and Shared projects compile without the experimental extension projects.
• Experimental (2.4.1-experimental): the Video Scaler and Image Foreground Extractor extension assemblies compile and are included.
• Switching from experimental back to stable removes stale extension outputs.

Packaging reaches the existing certificate-signing requirement (SigningCertificateNoCertSpecified) after successful C# compilation.

Out of scope

App Content Search, Conditional XAML Predicate, and the CMake samples are not included because they require API or package-support migration rather than source-tree consolidation.