URI loaded Images with ReactImageBrush do not consistently set Background brush when cached (when using clang)

Problem Description

XAML uses an internal memory and disk cache for images. This is true of BitmapImage as well as LoadedImageCache. In the case of LoadedImageCache, we can get into a situation where a network / URI image is pulled from cache and the LoadCompleted event subscribed here either does not fire or fires synchronously before we subscribe the event.

I suspect this is a clang-only issue, as this does not repro when building with MSVC, and we've seen other instances of clang behaving slightly differently w.r.t. coroutines (see #10772)

When this cached image issue occurs, we wind up not setting the image because we set the ReactImage background to the composition brush here.

Steps To Reproduce

Would require an environment where we build Microsoft.ReactNative with Clang in open source, which we do not have :(

Expected Results

No response

CLI version

npx react-native --version

Environment

npx react-native info

Target Platform Version

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Target Device(s)

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Visual Studio Version

No response

Build Configuration

No response

Snack, code example, screenshot, or link to a repository

No response

Start in vnext/Microsoft.ReactNative/Views/Image/ReactImage.cpp, especially the LoadCompleted subscription around line 266 and background assignment around line 292. Trace cached URI image handling and compare synchronous versus asynchronous completion under Clang. Done means cached images consistently set the ReactImage background brush, though the issue provides no reproducible Clang environment or named test.