Add response.content_type checks for JSON to api/v1 request specs (#31981)

spec/requests/api/v1/instances/languages_spec.rb

@@ -10,6 +10,8 @@ RSpec.describe 'Languages' do
 
     it 'returns http success and includes supported languages' do
       expect(response).to have_http_status(200)
+      expect(response.content_type)
+        .to start_with('application/json')
       expect(response.parsed_body.pluck(:code)).to match_array LanguagesHelper::SUPPORTED_LOCALES.keys.map(&:to_s)
     end
   end