Fix Attachment#save for binary IO

[Gujiassh]

Summary

• make Attachment#save rewind IO sources and write them in binary mode
• add regression coverage to ensure binary payloads are saved byte-for-byte without an added newline

Testing

• bundle exec rspec spec/ruby_llm/attachment_spec.rb
• bundle exec rubocop lib/ruby_llm/attachment.rb spec/ruby_llm/attachment_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.