Fix agent not propagating assume_model_exists from class config

[jeffmcfadden]

What this does

Fixes #679

Fix Agent.find not propagating assume_model_exists from class-level config.

This is most likely to be encountered when attempting to "continue" a previous chat with an Agent.

Agent.find loads a chat record from the database and then calls apply_configuration, which triggers to_llm. to_llm creates a new RubyLLM::Chat using the record's assume_model_exists attribute — but since this is a transient attr_accessor, it defaults to nil/false after an ActiveRecord load. This causes Models.resolve to raise ModelNotFoundError for models not in the registry, even when the agent class declares assume_model_exists: true.

with_rails_chat_record (used by create/create!) doesn't have this problem because it spreads **chat_kwargs into the record constructor, which includes assume_model_exists: true:

record = resolved_chat_model.public_send(method_name, **chat_kwargs, **chat_options)

The find path was missing the equivalent propagation. This fix adds it by setting assume_model_exists from chat_kwargs on the loaded record before apply_configuration runs.

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

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: #___

PRs for new features or enhancements without a prior approved issue will be closed.

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)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

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

[crmne]

Thanks for catching and fixing this.

I added a small follow-up to cover the remaining reload paths where assume_model_exists could still be dropped, plus regression specs so those paths stay protected.

Really appreciate the original report and fix.