code: def proxy_fetch(self, env, url):
          
          if not self.is_proxy_enabled(env):
              # we are not in proxy mode so just respond with forbidden
              return WbResponse.text_response('proxy mode must be enabled to use this endpoint',
                                              status='403 Forbidden')
  
          if env.get('REQUEST_METHOD') == 'OPTIONS':
              return WbResponse.options_response(env)
  
          # ensure full URL
          request_url = env['REQUEST_URI']
          # replace with /id_ so we do not get rewritten
          url = request_url.replace('/proxy-fetch', '/id_')
          # update WSGI environment object
          env['REQUEST_URI'] = self.proxy_coll + url
          env['PATH_INFO'] = env['PATH_INFO'].replace('/proxy-fetch', self.proxy_coll + '/id_')
          # make request using normal serve_content
          response = self.serve_content(env, self.proxy_coll, url)
          # for WR
          if isinstance(response, WbResponse):
              response.add_access_control_headers(env=env)
          return response
docs: Proxy mode only endpoint that handles OPTIONS requests and COR fetches for Preservation Worker.
  
          Due to normal cross-origin browser restrictions in proxy mode, auto fetch worker cannot access the CSS rules
          of cross-origin style sheets and must re-fetch them in a manner that is CORS safe. This endpoint facilitates
          that by fetching the stylesheets for the auto fetch worker and then responds with its contents
  
          :param dict env: The WSGI environment dictionary
          :param str url:  The URL of the resource to be fetched
          :return: WbResponse that is either response to an Options request or the results of fetching url
          :rtype: WbResponse