Support RubyLLM 1.16

[khasinski]

Summary

• add RubyLLM 1.16 compatibility for request instrumentation, stream tool-call accumulation, provider config options, and tool parallel-control capability
• keep RubyLLM 1.15 compatibility by guarding the new instrumentation API
• bump the gem version to 0.6.1 and document the release in the changelog

Validation

• mise exec -- bundle exec rspec
• mise exec -- bundle exec rubocop lib/ruby_llm/providers/openai_responses.rb lib/ruby_llm/providers/openai_responses/capabilities.rb spec/ruby_llm/providers/openai_responses_spec.rb spec/ruby_llm/providers/openai_responses/capabilities_spec.rb
• forced RubyLLM 1.15 smoke test
• ruby -c ruby_llm-responses_api.gemspec && ruby -c lib/rubyllm_responses_api.rb
• gem build ruby_llm-responses_api.gemspec