trim chat prompt based on llm context size

[BruceMacD]

When trimming the input chat prompt we need to make sure we keep the prompt template in the expected format. Without this the prompt will be trimmed without accounting for the model template when the maximum context length is reached, which can result in unexpected behavior from the model.

• update the ChatPrompt function to return a list of prompt variable, to allow the calling function to append them into the final prompt
• create the final prompt based on the loaded LLM's context window size, while preserving the prompt template formatting and system message in the first message of the new context window