feat(component): add ChatLLM binding for chat conversations

- Add ChatLLM binding to support separate LLM model for chat
- Update CallLLMNode to use ChatLLM instead of CheapLLM
- Update test fixtures to bind ChatLLM in all configurations
- Update unit tests to include ChatLLM binding

This enables 3 distinct LLM models:
- SmartLLM: Query generation from scratch
- CheapLLM: Query generation from cached samples
- ChatLLM: Chat conversations

Author: sf-sahil-jassal