This function takes a data frame and converts it into an LLMMessage object
representing a conversation history. The data frame should contain specific
columns (role and content) with each row representing a message in the
conversation.
df_llm_message(.df)An LLMMessage object containing the structured messages as per the input data frame.
A data frame with at least two rows and columns role and content.
The column role should contain values from "user", "assistant", or "system",
and content should be of type character.