Remove assumptions that XamlRoot::Content() matches Window size

Problem Description

There are at least two callsites that assume that the XamlRoot Size or the ActualSize of the XamlRoot::Content matches the size of the XAML Islands HWND:

• https://github.com/microsoft/react-native-windows/blob/003602b90aeda1b2a43702c6a9307c0f01010e59/vnext/Microsoft.ReactNative/Modules/LogBoxModule.cpp#L48-L54
• https://github.com/microsoft/react-native-windows/blob/cc7095ce487625a9a0a9b99b4b8df4043698eb78/vnext/Microsoft.ReactNative/Modules/AlertModule.cpp#L67-L85

While this is generally a good assumption, this may not always be the case. For example, in our app, we've worked around a XAML Islands bug https://github.com/microsoft/microsoft-ui-xaml/issues/2101 by making the root Grid larger than the HWND by a factor equivalent to the DPI scale factor.

Steps To Reproduce

There's no immediate repro in this repo, but one way to repro is to try and build a drag and drop handler for a XAML Islands react-native-windows app, and see that the full window is not a valid drop target for high DPI scale settings.

Expected Results

React Native Windows may want to assume that the root level content or XamlRoot matches the HWND size by default, but we should allow apps to override this behavior with a "window sizing root" target element that does match the window size.

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

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Build Configuration

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

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Start by inspecting the cited callsites in vnext/Microsoft.ReactNative/Modules/LogBoxModule.cpp and AlertModule.cpp, focusing on their use of XamlRoot size or XamlRoot::Content(). Define how an overridable window-sizing root target should work, then verify that both callsites no longer require root content or XamlRoot dimensions to match the HWND size.