Open pooled push connections as a Net::HTTP subclass that includes Stages

WebPush::Connections mixed Stages into each new connection with
`http.extend Stages` just before starting it. The connection is built in
one place, WebPush::PersistentRequest, so it can be built from a class
that already includes Stages: the same method lookup, without giving
every pooled connection its own singleton class. Connections#request
now also refuses a connection that isn't one, so a caller that builds a
plain Net::HTTP gets the ArgumentError rather than a connection whose
failures can't be told apart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_helpers/push_service_test_helper.rb

@@ -109,7 +109,7 @@ module PushServiceTestHelper
 
     # Built as WebPush::PersistentRequest builds it for a delivery.
     def pinned_connection(server, ip = IP)
-      Net::HTTP.new(HOST, server.port, nil).tap do |http|
+      WebPush::Connections::HTTP.new(HOST, server.port, nil).tap do |http|
         http.ipaddr = ip
         http.use_ssl = true
       end