Ruby 3.5 compatibility: add cgi gem, drop runtime rack‑test

[numbata]

Ruby 3.5 retires the cgi library from the default gem set (ruby/ruby#13275), so we add it explicitly. Also the rack‑test gem is only used in specs, so it’s been dropped from the gemspec’s runtime deps; it remains in the dev/test Gemfile group.

[dblock]

Let's use this to fix up CHANGELOG?

Also maybe re-enable danger PR bot like in other grape projects ...

[numbata]

Also maybe re-enable danger PR bot like in other grape projects ...

Sure. I can do it in a follow up PR.

[numbata]

As an option, we can close this PR and wait for the next Rack release that includes rack/rack#2327 (“Replace usage of CGI::Cookie; add Ruby 3.5 compatibility”).

Why it fails today (require chain)

spec/spec_helper.rb             → require 'rack/test'
rack-test/lib/rack/test.rb      → require 'rack/mock'
rack/lib/rack/mock.rb           → require_relative 'mock_request'
rack/lib/rack/mock_request.rb   → require_relative 'mock_response'
rack/lib/rack/mock_response.rb  → require 'cgi/cookie'   # removed in Ruby 3.5

The patch removes the offending require 'cgi/cookie' line, so the failure disappears once we bump Rack. Until that release is out, CI for all PR will stay 🔴.

[dblock]

I merged it.