fix(audience-tests): exclude Unity/ tests from headless dotnet build (SDK-296)

DeviceCollectorTests references Immutable.Audience.Unity which wraps
UnityEngine APIs not available outside the Unity runtime. Exclude
Runtime/Unity/ from the csproj the same way Editor/ is already excluded;
the tests run via the Unity Test Framework through the asmdef instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nattb8

src/Packages/Audience/Tests/Audience.Tests.csproj

@@ -22,5 +22,9 @@
   -->
   <ItemGroup>
     <Compile Remove="Editor/**/*.cs" />
+    <!-- DeviceCollectorTests references Immutable.Audience.Unity which wraps
+         UnityEngine APIs unavailable in the headless dotnet build. These tests
+         run inside the Unity Test Framework via the asmdef instead. -->
+    <Compile Remove="Runtime/Unity/**/*.cs" />
   </ItemGroup>
 </Project>