Add Chat#step single-iteration execution

[Gujiassh]

Summary

• add Chat#step to run exactly one provider iteration plus any resulting tool execution without recursing through the full agent loop
• refactor complete to delegate to step so existing behavior stays unchanged while Tool::Halt semantics are preserved
• add focused chat tool regressions proving step executes one tool-calling turn and returns Tool::Halt immediately when a halting tool is invoked

Validation

• bundle exec rspec spec/ruby_llm/chat_tools_spec.rb --example 'step executes a single tool-calling iteration without recursing' --example 'step returns Halt when a tool halts'
• bundle exec rspec spec/ruby_llm/chat_tools_spec.rb
• bundle exec rubocop lib/ruby_llm/chat.rb spec/ruby_llm/chat_tools_spec.rb

[Gujiassh]

Closing this stale radar PR to keep the queue clean: it has been open for weeks without maintainer response, and I am not going to keep it in the active follow-up set without renewed upstream interest.