Close persistent proxy clients in dependency order

[samuel-williams-shopify]

Summary

• add a regression test using a real local HTTP server and CONNECT proxy
• close cached clients in reverse insertion order so tunnelled clients close before the proxy clients they depend on

A CONNECT tunnel remains an active request on the proxy client until the tunnelled client closes. Client#close waits for active requests to drain, but PersistentClients inserted the proxy client before the tunnelled client and then closed in insertion order. That made shutdown wait on the CONNECT request before closing the client that could finish it.

This addresses the deterministic persistent-client shutdown deadlock isolated while investigating #59.

Verification

• regression test fails on the first commit with Async::TimeoutError in Async::Pool::Controller#wait_until_free
• bundle exec sus — 28 examples, 138 assertions
• bundle exec rubocop lib/async/http/faraday/clients.rb test/async/http/faraday/clients.rb