fix(tests): configure golden test data to copy to output directory

Previously, the golden-mappings.json file in the TestData directory was not being copied to the output directory, causing golden tests to skip silently when the file couldn't be found at runtime.

Added ItemGroup configuration to Umbraco.Tests.UnitTests.csproj to copy all JSON files from Umbraco.Core\Strings\TestData\ to the output directory using PreserveNewest, ensuring the golden test data is available for test execution.

Generated with [Claude Code](https://claude.com/claude-code)

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

tests/Umbraco.Tests.UnitTests/Umbraco.Tests.UnitTests.csproj

@@ -51,4 +51,10 @@
       <DependentUpon>ContentExtensionsTests.cs</DependentUpon>
     </Compile>
   </ItemGroup>
+
+  <ItemGroup>
+    <None Include="Umbraco.Core\Strings\TestData\*.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
 </Project>