- Add model_manager parameter to LiteLLMBackend for alias resolution
- Add _resolve_model_alias() method to handle default, image, audio, tts aliases
- Update get_litellm_backend() to pass model_manager
- Update coderai call site to pass multi_model_manager

Now --parser litellm will resolve aliases like 'default', 'image' to actual model names before normalizing for litellm.