Exclude snippets from external caching handling

Fixes https://gitlab.com/gitlab-org/gitlab/issues/193498

Proper handling would be tackled separately in
https://gitlab.com/gitlab-org/gitlab/issues/194118.

app/helpers/application_helper.rb

@@ -198,7 +198,7 @@ module ApplicationHelper
   end
 
   def external_storage_url_or_path(path, project = @project)
-    return path unless static_objects_external_storage_enabled?
+    return path if @snippet || !static_objects_external_storage_enabled?
 
     uri = URI(Gitlab::CurrentSettings.static_objects_external_storage_url)
     path = URI(path) # `path` could have query parameters, so we need to split query and path apart

changelogs/unreleased/fix-skip-snippet-raw-buttons-for-external-caching.yml

+---
+title: Exclude snippets from external caching handling
+merge_request:
+author:
+type: fixed

spec/helpers/application_helper_spec.rb

@@ -206,6 +206,15 @@ describe ApplicationHelper do
       end
     end
 
+    context 'when @snippet is set' do
+      it 'returns the passed path' do
+        snippet = create(:snippet)
+        assign(:snippet, snippet)
+
+        expect(helper.external_storage_url_or_path('/foo/bar', project)).to eq('/foo/bar')
+      end
+    end
+
     context 'when external storage is enabled' do
       let(:user) { create(:user, static_object_token: 'hunter1') }