Update Langchain execution to LangGraph

[Linked-Liszt]

LangGraph is the way that LangChain v0.3 recommends creating action sequences.

https://python.langchain.com/docs/versions/migrating_memory/conversation_buffer_window_memory/#modern-usage-with-langgraph

It is much more flexible but will require a major refactor of all langchain callables in the program. This must be done as the old API will be deprecated in LangChain 1.0 forcing all users to use LangGraph.