Add rdoc gem for Ruby 4.0 compatibility

Ruby 4.0 removed rdoc from the default gems, so the unconditional
`require "rdoc/task"` in rakefile:45 raises LoadError when running
`bundle exec rake spec` under the devcontainer's Ruby 4.0.3. Declaring
rdoc in the development group keeps the rdoc-related rake tasks
available without changing the gemspec or restructuring the rakefile.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

Gemfile

@@ -2,6 +2,7 @@ source "https://rubygems.org"
 
 group :development do
   gem "rspec_junit_formatter"
+  gem "rdoc"
 end
 
 group :rubocop do