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/pdf_edit.py

@@ -6,12 +6,13 @@ from pydantic import Field
 
 from stirling.models import ApiModel
 
-from .common import ToolOperationStep, WorkflowOutcome
+from .common import ConversationMessage, ToolOperationStep, WorkflowOutcome
 
 
 class PdfEditRequest(ApiModel):
     user_message: str
     file_names: list[str] = Field(default_factory=list)
+    conversation_history: list[ConversationMessage] = Field(default_factory=list)
 
 
 class EditPlanResponse(ApiModel):