Fix/canvas gallery

[Drout]

Summary

This PR contains two related changes:

1. Fixes canvas gallery item typing so gallery item access resolves child controls correctly in published apps.
2. Retargets PowerAppsTestEngineWrapper to .NET 8 to eliminate warnings and align with the current dependency set.

What changed

Canvas gallery item support

• Added RunJavascriptInFrameAsync<T> to the test infra so the canvas provider can evaluate JavaScript directly inside the published app iframe.
• Updated the published app JS object-model builder to reconstruct gallery AllItems / Items and Selected / Default property types from gallery child controls.
• Added a provider-side fallback that patches gallery child typing when the runtime still reports empty collection/object shapes.

This enables expressions that depend on gallery item child controls, such as indexing into gallery items and then accessing nested control properties.

Wrapper target framework update

• Changed PowerAppsTestEngineWrapper from netstandard2.0 to net8.0.
• Removed package settings that were only needed for the previous target/warning profile.

Validation

• Built src/PowerAppsTestEngine.sln successfully in Debug configuration with 0 warnings and 0 errors.

[Drout]

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