I don't think we need to cover this, if Gemfile.in is found, it is likely bundler_ext is there. Otherwise why whould user have Gemfile.in file? You only see those when developing and the message is something like:

   Cannot require: bundler_ext

But again, users will not be hitting this. If we implement a fallback and user have some deviations in their setup, it can happen that they will be running Foreman in the fallback mode for years without noticing, which can be dangerous.