✅ add spec for default_collection_image in `ThumbnailPathServiceDec…

…orator`

- Added a test for `default_collection_image` to verify it returns the site-specific default collection image when available.
- Stubbed `Site.instance` to ensure the spec accurately tests behavior when a site default image is present or absent.

spec/services/hyrax/thumbnail_path_service_spec.rb

@@ -18,4 +18,26 @@
       end
     end
   end
+
+  describe '.default_collection_image' do
+    context 'when the site has a default collection image' do
+      let(:collection_image) { '/assets/site_default_collection_image.png' }
+      let(:site_instance_double) { instance_double(Site, default_collection_image: double('DefaultCollectionImage', url: collection_image)) }
+
+      before do
+        # Stub Site.instance to return our site_instance_double with the expected url
+        allow(Site).to receive(:instance).and_return(site_instance_double)
+      end
+
+      it 'returns the default collection image from the site' do
+        expect(described_class.default_collection_image).to eq(collection_image)
+      end
+    end
+
+    context 'when the site does not have a default collection image' do
+      it 'returns the Hyrax default collection image' do
+        expect(described_class.default_collection_image).to eq(ActionController::Base.helpers.image_path('default.png'))
+      end
+    end
+  end
 end