Add DefaultDllImportSearchPaths to WindowsAppRuntime_EnsureIsLoaded

[guimafelipe]

Summary

• Adds [DefaultDllImportSearchPaths(DllImportSearchPath.AssemblyDirectory | DllImportSearchPath.System32)] to the WindowsAppRuntime_EnsureIsLoaded P/Invoke declaration
• Constrains DLL search to assembly directory and System32, suppressing CA5392 code analysis warning
• Consistent with security best practices for DLL loading

Fixes #4857