feat(llm): add LangChain adapter and framework registry (#1759)

Part of the LangChain decoupling stack:
- `LangChainLLMAdapter` wrapping `BaseChatModel` behind `LLMModel` protocol
- `LangChainFramework` implementing `create_model()`
- Framework registry with `register_framework()` / `get_framework()` / `set_default_framework()`
- `chatmessage_to_langchain_message()` and `chatmessages_to_langchain_messages()` conversion utils
- `provider_url` extracts endpoint from LangChain LLM attributes

Author: Pouyanpi