Return 404 when the iiif endpoint is called for a file on a collection

Filxes #1152

app/models/stacks_rights.rb

@@ -59,9 +59,11 @@ def cocina_file
   end
 
   def find_file
-    public_json.dig('structural', 'contains')
-               .lazy.flat_map { |file_set| file_set.dig('structural', 'contains') }
-               .find { |file| file['filename'] == file_name } || raise(ActionController::MissingFile, "File not found '#{file_name}'")
+    file_sets = public_json.dig('structural', 'contains')
+    raise(ActionController::MissingFile, "File not found '#{file_name}'") unless file_sets # Trap for Collections
+
+    file_sets.lazy.flat_map { |file_set| file_set.dig('structural', 'contains') }
+             .find { |file| file['filename'] == file_name } || raise(ActionController::MissingFile, "File not found '#{file_name}'")
   end
 
   def public_json

spec/requests/iiif_spec.rb

@@ -265,17 +265,6 @@
   end
 
   describe 'image requests for world readable items' do
-    let(:ability) { instance_double(CocinaAbility, can?: false) }
-
-    before do
-      # for the cache headers
-      allow_any_instance_of(IiifController).to receive(:anonymous_ability).and_return ability
-      # for authorize! in #show
-      allow_any_instance_of(IiifController).to receive(:authorize!).with(:read, Projection).and_return(true)
-      # for current_image
-      allow_any_instance_of(IiifController).to receive(:can?).with(:download, StacksImage).and_return(true)
-    end
-
     context 'when the request is valid' do
       before do
         stub_request(:get, "http://imageserver-prod.stanford.edu/iiif/2/#{image_server_path('nr349ct7889', 'image.jp2')}/0,640,2552,2552/100,100/0/default.jpg")
@@ -319,6 +308,19 @@
       end
     end
 
+    context 'when the object has no filesets (e.g. a collection)' do
+      let(:public_json) do
+        { 'structural' => {} }
+      end
+
+      it 'returns 404 Not Found' do
+        stub_request(:get, "http://imageserver-prod.stanford.edu/iiif/2/nr%2F349%2Fct%2F7889%2Fimage.jp2/0,640,2552,2552/100,100/0/default.jpg")
+          .to_return(status: 200, body: "", headers: {})
+        get "/image/iiif/nr349ct7889%2Fimage.jp2/0,640,2552,2552/100,100/0/default.jpg"
+        expect(response).to have_http_status :not_found
+      end
+    end
+
     context 'with the download flag set' do
       before do
         stub_request(:get, "http://imageserver-prod.stanford.edu/iiif/2/#{image_server_path('nr349ct7889', 'image.jp2')}/0,640,2552,2552/100,100/0/default.jpg")