Consolidate Langchain message to OpenAI message conversion implementations

[edwardzjl]

It appears there are currently two distinct implementations for converting Langchain messages to OpenAI format messages:

• langchain_core.messages.utils.convert_to_openai_messages:
  

  langchain/libs/core/langchain_core/messages/utils.py

  Line 941 in ce0b1a9

  def convert_to_openai_messages(

• langchain_openai.chat_models.base._convert_message_to_dict:
  

  langchain/libs/partners/openai/langchain_openai/chat_models/base.py

  Line 234 in ce0b1a9

  def _convert_message_to_dict(message: BaseMessage) -> dict:

The private function _convert_message_to_dict is currently used internally in three places within langchain_openai.chat_models.base.py:

langchain/libs/partners/openai/langchain_openai/chat_models/base.py

Line 985 in ce0b1a9

payload["messages"] = [_convert_message_to_dict(m) for m in messages]

langchain/libs/partners/openai/langchain_openai/chat_models/base.py

Line 1279 in ce0b1a9

messages_dict = [_convert_message_to_dict(m) for m in messages]

langchain/libs/partners/openai/langchain_openai/chat_models/base.py

Line 3107 in ce0b1a9

msg = _convert_message_to_dict(lc_msg)

Having two separate functions performing the same core conversion logic can lead to inconsistencies, potential bugs, and increased maintenance overhead.

Based on the docstring, langchain_core.messages.utils.convert_to_openai_messages appears to be the intended standard or newer public utility function for this conversion.

Suggestion:

It is suggested to consolidate these implementations. We should migrate the internal usages of _convert_message_to_dict within langchain_openai to use langchain_core.messages.utils.convert_to_openai_messages instead. After migrating the usages, the private _convert_message_to_dict function could potentially be removed or deprecated.

This consolidation would ensure a single source of truth for message conversion and simplify the codebase.