Create the intermediate directory before zipping test assets

ZipTestAssets runs BeforeTargets="PrepareForBuild", but PrepareForBuild is
what creates obj/$(Configuration)/$(TargetFramework) - so on a clean tree
ZipDirectory failed with MSB3943 because its destination directory did not
exist yet. Only showed up in CI, where the Release obj folder is never
already there from an earlier build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MFrWcjXmpfZqDNz63MEKfW

Author: aritchie

tests/Shiny.Net.HttpServer.Tests/Shiny.Net.HttpServer.Tests.csproj

@@ -62,6 +62,9 @@
           BeforeTargets="PrepareForBuild"
           Inputs="@(TestAssetFile)"
           Outputs="$(IntermediateOutputPath)Assets.zip">
+    <!-- PrepareForBuild is what creates obj/$(Configuration)/$(TargetFramework), and this runs
+         before it - so on a clean tree the directory the archive goes in does not exist yet. -->
+    <MakeDir Directories="$(IntermediateOutputPath)" />
     <ZipDirectory SourceDirectory="$(MSBuildThisFileDirectory)TestAssets"
                   DestinationFile="$(IntermediateOutputPath)Assets.zip"
                   Overwrite="true" />