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>

lib/web_push/connections.rb

@@ -26,6 +26,12 @@ class WebPush::Connections
       end
   end
 
+  # The Net::HTTP a pooled delivery is opened with: Stages is part of the class, so a connection carries its stage
+  # from the start instead of having the module mixed into each one as it's opened.
+  class HTTP < Net::HTTP
+    include Stages
+  end
+
   def initialize(keep_alive_timeout: 30, max_idle: 150)
     @keep_alive_timeout = keep_alive_timeout
     @max_idle = max_idle
@@ -35,7 +41,7 @@ class WebPush::Connections
   end
 
   def request(http, request)
-    unless http.ipaddr && !http.proxy? && http.use_ssl? && !http.started?
+    unless http.is_a?(HTTP) && http.ipaddr && !http.proxy? && http.use_ssl? && !http.started?
       raise ArgumentError, "Only new, direct TLS connections pinned to an address are pooled"
     end
     address = [ http.address, http.port, http.ipaddr ]
@@ -48,7 +54,6 @@ class WebPush::Connections
       end
     end
 
-    http.extend Stages
     http.keep_alive_timeout = @keep_alive_timeout
     http.start
     send_over(http, request, reused: false).tap { checkin(address, http) }