perf: set content attribute optional to AssistantMessageSegment for enhanced message handling

fixes: #3843

astrbot/core/agent/message.py

@@ -153,6 +153,7 @@ class AssistantMessageSegment(Message):
     """A message segment from the assistant."""
 
     role: Literal["assistant"] = "assistant"
+    content: str | list[ContentPart] | None = None
     tool_calls: list[ToolCall] | list[dict] | None = None