Fix minitest compatibility with Ruby 3.x

Remove `require 'minitest/unit'` which was removed in recent minitest
versions, and use `Minitest::Assertions` instead of `MiniTest::Assertions`.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dblock

test/test_helper.rb

@@ -17,14 +17,13 @@
 
 require 'minitest/autorun'
 require 'minitest/pride'
-require 'minitest/unit'
 require 'mocha/minitest'
 require 'json'
 
 require 'faraday'
 require 'faraday/request/instrumentation'
 
-MiniTest::Assertions.class_eval do
+Minitest::Assertions.class_eval do
   def stub_request(conn, adapter_class = Faraday::Adapter::Test, &stubs_block)
     adapter_handler = conn.builder.handlers.find { |h| h.klass < Faraday::Adapter }
     if adapter_handler