Add ResetMessage to clear the agent state (microsoft#3988)

* Reset message to clear agent state

* format and lint

python/packages/autogen-agentchat/src/autogen_agentchat/agents/_assistant_agent.py

@@ -21,6 +21,7 @@
 from ..messages import (
     ChatMessage,
     HandoffMessage,
+    ResetMessage,
     StopMessage,
     TextMessage,
 )
@@ -209,7 +210,10 @@ def __init__(
     async def on_messages(self, messages: Sequence[ChatMessage], cancellation_token: CancellationToken) -> ChatMessage:
         # Add messages to the model context.
         for msg in messages:
-            self._model_context.append(UserMessage(content=msg.content, source=msg.source))
+            if isinstance(msg, ResetMessage):
+                self._model_context.clear()
+            else:
+                self._model_context.append(UserMessage(content=msg.content, source=msg.source))
 
         # Generate an inference result based on the current model context.
         llm_messages = self._system_messages + self._model_context

python/packages/autogen-agentchat/src/autogen_agentchat/messages.py

@@ -42,7 +42,14 @@ class HandoffMessage(BaseMessage):
     """The handoff message to the target agent."""
 
 
-ChatMessage = TextMessage | MultiModalMessage | StopMessage | HandoffMessage
+class ResetMessage(BaseMessage):
+    """A message requesting reset of the recipient's state in the current conversation."""
+
+    content: str
+    """The content for the reset message."""
+
+
+ChatMessage = TextMessage | MultiModalMessage | StopMessage | HandoffMessage | ResetMessage
 """A message used by agents in a team."""
 
 
@@ -52,5 +59,6 @@ class HandoffMessage(BaseMessage):
     "MultiModalMessage",
     "StopMessage",
     "HandoffMessage",
+    "ResetMessage",
     "ChatMessage",
 ]