Update for Ruby 4.0, Puma 7+, and minitest

[henrikbjorn]

Summary

• Use native Puma::Server API instead of deprecated Rack::Handler::Puma
• Require Ruby >= 4.0.0, Sinatra >= 4.0, Puma >= 7.0
• Convert tests from Test::Unit to minitest with proper teardown methods
• Add Rakefile with rake test task
• Add GitHub Actions CI workflow for Ruby 4.0

Test plan

• Run bundle install to install dependencies
• Run bundle exec rake to execute tests
• Verify CI passes on GitHub Actions