Allow chat history to be sent to AI engine (#6128)

# Description of Changes
Add an extra parameter to every agent to receive the conversation
history in addition to the current message. This will make it possible
to answer followup questions from the AI without needing to give full
context in your message.

engine/src/stirling/contracts/common.py

@@ -91,6 +91,12 @@ class ConversationMessage(ApiModel):
     content: str
 
 
+def format_conversation_history(conversation_history: list[ConversationMessage]) -> str:
+    if not conversation_history:
+        return "None"
+    return "\n".join(f"- {message.role}: {message.content}" for message in conversation_history)
+
+
 class PdfTextSelection(ApiModel):
     page_number: int | None = None
     text: str