messagesStateReducer

Reducer function for combining two sets of messages in LangGraph's state system.

This reducer handles several tasks:

1. Normalizes both left and right message inputs to arrays.
2. Coerces any message-like objects into real BaseMessage instances.
3. Ensures all messages have unique, stable IDs by generating missing ones.
4. If a RemoveMessage instance is encountered in right with the ID REMOVE_ALL_MESSAGES, all previous messages are discarded and only the subsequent messages in right are returned.
5. Otherwise, merges left and right messages together following these rules:
   • If a message in right shares an ID with a message in left:
     • If it is a RemoveMessage, that message (by ID) is marked for removal.
     • If it is a normal message, it replaces the message with the same ID from left.
   • If a message in right does not exist in left:
     • If it is a RemoveMessage, this is considered an error (cannot remove non-existent ID).
     • Otherwise, the message is appended.
   • Messages flagged for removal are omitted from the final output.