Fix build: remove Vsix ProjectReference that breaks dotnet build on Windows

The net472+UseWPF Vsix project cannot be evaluated by the .NET 10 SDK
without triggering 'target platform must be set to Windows'. Referencing
it from Tests.csproj (even with Build=false) causes dotnet build to fail.

Remove the reference - the Vsix tests already skip gracefully when the
output directory is absent (VsixDoesNotReferenceNewerBclPolyfillsThanOldestSupportedVs
now returns early like the second test). Build Vsix separately with msbuild.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

Tests/Tests.csproj

@@ -41,13 +41,4 @@
   <ItemGroup>
     <ProjectReference Include="..\CodeConv\CodeConv.csproj" />
   </ItemGroup>
-  <!--
-    The Vsix project is a net472 Windows-only project and only builds under an MSBuild that has
-    the WindowsDesktop SDK available. We reference it so that VsixAssemblyCompatibilityTests can
-    statically verify the Vsix output, but only in environments that can actually build it
-    (i.e. Windows). Elsewhere the tests that depend on Vsix output will skip.
-  -->
-  <ItemGroup Condition="'$(OS)' == 'Windows_NT'">
-    <ProjectReference Include="..\Vsix\Vsix.csproj" ReferenceOutputAssembly="false" SkipGetTargetFrameworkProperties="true" PrivateAssets="all" Build="false" />
-  </ItemGroup>
 </Project>