fix: repair Puma/Rack dep rot so the test suite boots

.github/workflows/test.yml

@@ -2,6 +2,7 @@ name: Test
 on:
   push:
     branches: [main]
+  pull_request:
   workflow_dispatch:
     inputs:
       branch:

.gitignore

@@ -16,3 +16,4 @@ mkmf.log
 Gemfile.lock
 node_modules/
 .venv/
+/vendor/

Gemfile

@@ -7,8 +7,13 @@ gem "guard-rspec", require: false
 
 group :test, :development do
   gem "webmock"
-  gem "puma", '~> 6'
-  gem "rackup"
+  # Capybara 3.36 (the newest Capybara that supports Ruby 2.6, which CI still
+  # targets) uses the Puma 5 events API; Puma 6 removed Puma::Events.strings,
+  # which broke the Capybara server boot. Pin to Puma 5 for compatibility.
+  gem "puma", '~> 5'
+  # Puma 5's rack handler requires `rack/handler`, which Rack 3 removed (it
+  # moved to the separate `rackup` gem). Pin Rack 2 so Capybara can boot Puma.
+  gem "rack", '~> 2.2'
   gem "pry"
   gem "simplecov", require: false
 end