Preserves assume_model_exists in to_llm for custom models

[creaumond]

What this does

When a user passes in the assume_model_exists flag through a custom Rails model, the flag gets dropped and they are not able to utilize a custom model.

This persists that flag or sets it to false if not provided.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

Related issues

#555

[schoblaska]

Confirmed that this fix works on my test repo. Thanks, @creaumond !

ruby_llm_test $ OPENROUTER_API_KEY=sk-or-v1-asdf1234 bin/rake test_ruby_llm
Testing RubyLLM.chat with unknown model
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Testing Llm::Chat with known model
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Testing Llm::Chat with unknown model
Hi there! How can I help you today?