fix: configure xunit to disable test parallelization

Configure xunit.runner.json to disable both assembly and test collection
parallelization to prevent intermittent test failures in Release mode.
Also update Graphify.Tests.csproj to include xunit.runner.json in output.

Tests now run sequentially without interfering with each other's shared
state (particularly the appsettings.local.json file used by config tests).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

src/tests/Graphify.Tests/Graphify.Tests.csproj

@@ -19,4 +19,8 @@
     <ProjectReference Include="..\..\Graphify.Cli\Graphify.Cli.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <None Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
 </Project>