feat: Ensure that uno.sdk winappsdk apps include all Assets by default

src/SolutionTemplate/5.2/uno52blank/uno52blank/uno52blank.csproj

@@ -28,4 +28,13 @@
     <!-- <WindowsSdkPackageVersion>10.0.22621.28</WindowsSdkPackageVersion> -->
   </PropertyGroup>
 
+  <Target Name="_UnoValidateSelfAssets" 
+          BeforeTargets="AfterBuild" 
+          Condition="'$(TargetFramework)'=='$(DotNetVersion)-windows10.0.19041'">
+
+    <!-- Validates that assets are properly propagated to the output folder, regardless of their "copy to output" value -->
+    <Error Text="Missing asset $(OutputPath)\Assets\SharedAssets.md"
+          Condition="!exists('$(OutputPath)\Assets\SharedAssets.md')" />
+  </Target>
+
 </Project>

src/Uno.Sdk/targets/Uno.Common.WinAppSdk.targets

@@ -46,6 +46,13 @@
 		<Manifest Include="$(ApplicationManifest)" Exclude="@(Manifest)" />
 	</ItemGroup>
 
+	<ItemGroup>
+		<!-- Replicates the original globbing from WinAppSDK (See microsoft.windowsappsdk\1.4.240211001\buildTransitive\Microsoft.WinUI.props)-->
+		<Content Update="Assets/**/*.*" Condition="'$(EnableDefaultAssets)'=='true'">
+			<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+		</Content>
+	</ItemGroup>
+
 	<!-- Workaround to avoid including Project XBFs in the PRI file: https://github.com/microsoft/microsoft-ui-xaml/issues/8857 -->
 	<Import Project="winappsdk-workaround.targets"
 			Condition=" '$(SingleProject)' != 'true' and '$(DisableWinUI8857_Workaround)' != 'true' "/>

src/Uno.Sdk/targets/Uno.SingleProject.WinAppSdk.targets

@@ -30,6 +30,10 @@
 			Condition="Exists('$(ApplicationManifest)')" />
 	</ItemGroup>
 
+	<ItemGroup>
+		<Content Update="" />
+	</ItemGroup>
+
 	<!-- Using .NET MAUI Workaround for AnyCPU https://github.com/dotnet/maui/blob/37ca0c26e39a40f14b4fd60416b90a1a62e28473/src/Workload/Microsoft.Maui.Sdk/Sdk/Microsoft.Maui.Sdk.Before.targets#L12-L22 -->
 	<!-- The Windows App SDK does not support AnyCPU without a RID, so make sure we pick the "best" one we can -->
 	<PropertyGroup>