refactor(srp): let universal assets declare native backend keys

managed/GameScripts/RenderPipelineApiProbe.cs

@@ -926,6 +926,20 @@ namespace Gameplay
         }
     }
 
+    public sealed class ManagedUnknownBackendRenderPipelineProbeAsset
+        : UniversalRenderPipelineAsset
+    {
+        public ManagedUnknownBackendRenderPipelineProbeAsset()
+        {
+            rendererData = new ManagedRenderPipelineProbeRendererData();
+        }
+
+        protected override string GetPipelineRendererAssetKey()
+        {
+            return "MissingBackend";
+        }
+    }
+
     public sealed class ManagedUniversalRenderPipelineProbeAsset
         : UniversalRenderPipelineAsset
     {