Fix AttributeError raised on retriever

[pamella]

Description

• Resolves RAG fail with TypeError: argument 'text': 'HumanMessage' object cannot be converted to 'PyString' #186

---

An AttributeError was happening within the retriever method during execution.

The issue happened at the line:

docs = retriever.invoke(
    {"input": input_message, "history": messages_to_summarize}
)

because a string was expected in for the input and not a HumanMessage. See langchain's retriever.invoke: https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/retrievers.py#L201

Steps to test

Screencast.from.19-11-2024.11.10.31.webm