Fix builds on WinUI and UWP .NET 9 projects (#15004)

* Fix builds on UWP .NET 9 projects

* Fix builds for WinUI as well

src/package/Microsoft.NET.Test.Sdk/netcoreapp/Microsoft.NET.Test.Sdk.targets

@@ -15,13 +15,22 @@
   <!--
     Output type for .NET Core test projects should be exe.
     https://devdiv.visualstudio.com/DevDiv/_workitems?id=375688&_a=edit
+    Note: we skip this for WinUI and UWP apps, as they must use 'WinExe'.
   -->
   <PropertyGroup>
-    <OutputType>Exe</OutputType>
+    <OutputType Condition="'$(UseWinUI)' != 'true' AND '$(UseUwpTools)' != 'true'">Exe</OutputType>
   </PropertyGroup>
 
   <PropertyGroup>
     <GeneratedProgramFile Condition="'$(GeneratedProgramFile)' == ''">$(MSBuildThisFileDirectory)Microsoft.NET.Test.Sdk.Program$(DefaultLanguageSourceExtension)</GeneratedProgramFile>
+
+    <!--
+      We always skip the generated entry point for UWP apps on .NET 9, as those need a separate entry point
+      to initialize the 'CoreApplication' app, which is handled separately. That is, it will either be in
+      the project template, or generated by the XAML compiler (for UWP XAML apps). We also do the same
+      for WinUI apps, since those also have a special entry point produced by the XAML compiler.
+  -->
+    <GenerateProgramFile Condition="'$(GenerateProgramFile)' == '' AND ('$(UseWinUI)' == 'true' OR '$(UseUwpTools)' == 'true')">false</GenerateProgramFile>
     <GenerateProgramFile Condition="'$(GenerateProgramFile)' == ''">true</GenerateProgramFile>
   </PropertyGroup>