Fix OpenAI search model temperature normalization

[Gujiassh]

Summary

• make OpenAI search models drop temperature before the broader gpt-5 normalization branch runs
• add regression coverage for gpt-5-search-api alongside the existing search model expectations

Testing

• bundle exec rspec spec/ruby_llm/providers/open_ai/temperature_spec.rb -e \"returns nil for search models\"
• bundle exec rspec spec/ruby_llm/providers/open_ai/temperature_spec.rb
• bundle exec rubocop lib/ruby_llm/providers/openai/temperature.rb spec/ruby_llm/providers/open_ai/temperature_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.