fix: add missing imports and correct agent initialization syntax (#1536)

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Author: yanglikun

src/oss/langchain/short-term-memory.mdx

@@ -964,6 +964,7 @@ from langgraph.graph.message import REMOVE_ALL_MESSAGES
 from langgraph.checkpoint.memory import InMemorySaver
 from langchain.agents import create_agent, AgentState
 from langchain.agents.middleware import before_model
+from langchain_core.runnables import RunnableConfig
 from langgraph.runtime import Runtime
 from typing import Any
 
@@ -987,11 +988,12 @@ def trim_messages(state: AgentState, runtime: Runtime) -> dict[str, Any] | None:
         ]
     }
 
+
 agent = create_agent(
-    model,
+    "gpt-5-nano",
     tools=[],
-    middleware=[trim_messages]
-    checkpoint=InMemorySaver()
+    middleware=[trim_messages],
+    checkpointer=InMemorySaver()
 )
 
 config: RunnableConfig = {"configurable": {"thread_id": "1"}}