Update Windows App SDK to 2.5.1 and its WinUI API surface #24979

What would you like to be added? 💡

Move the Windows App SDK references from 2.4.0 to the latest stable 2.5.1 (WinUI sub-package 2.3.6 → 2.3.9), and regenerate the WinUI sync generator API stubs against it:

• src/Directory.Packages.props (repo-internal heads, runtime tests, sync generator input)
• src/Uno.Sdk/packages.json (WinAppSdk consumer default)
• Uno.WinUI / Uno.WinUI.Graphics2DSK nuspec dependency floors (kept in lockstep with the Uno.Sdk default to avoid NU1605)
• The Windows App Runtime installer pinned in the WinUI runtime tests CI leg
• ResourcesExtractor, the 7.0 migration guide and the winui-runtime-tests skill notes

Why is this needed? 🤔

Runtime tests validate Uno Platform behavior against native WinUI, and the sync generator defines Uno's WinUI API surface from the referenced Windows App SDK. Staying on the latest stable keeps both aligned with current WinUI.

Renderer 🎨

• Skia
• Native

For which platforms 📱💻🖥️

• Windows App SDK
• Build tasks

Anything else we need to know? 💬

2.5.1 does not raise the WebView2 (1.0.3719.77) or Windows SDK BuildTools (10.0.26100.4654) floors. The only API surface delta picked up by the sync generator is three new Microsoft.Graphics.DirectX.DirectXColorSpace values.