chore(deps): align unit test framework with UITests

Bumps Microsoft.NET.Test.Sdk 17.12.0 → 18.5.1 and
xunit.runner.visualstudio 2.8.2 → 3.1.5 in the unit test project so
both test projects share the same test framework after #45.

All 82 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: AThraen

tests/CodeShellManager.Tests/CodeShellManager.Tests.csproj

@@ -6,9 +6,9 @@
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
     <PackageReference Include="xunit" Version="2.9.3" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>