fix: correct imports and create_agent construction (#1558)

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---------

Co-authored-by: Mason Daugherty <github@mdrxy.com>

Author: yanglikun

src/oss/langchain/middleware/custom.mdx

@@ -373,30 +373,36 @@ Middleware can extend the agent's state with custom properties.
 <Tab title="Decorator">
 
 ```python
+from langchain.agents import create_agent
+from langchain.messages import HumanMessage
 from langchain.agents.middleware import AgentState, before_model, after_model
 from typing_extensions import NotRequired
 from typing import Any
 from langgraph.runtime import Runtime
 
+
 class CustomState(AgentState):
     model_call_count: NotRequired[int]
     user_id: NotRequired[str]
 
+
 @before_model(state_schema=CustomState, can_jump_to=["end"])
 def check_call_limit(state: CustomState, runtime: Runtime) -> dict[str, Any] | None:
     count = state.get("model_call_count", 0)
     if count > 10:
         return {"jump_to": "end"}
     return None
 
+
 @after_model(state_schema=CustomState)
 def increment_counter(state: CustomState, runtime: Runtime) -> dict[str, Any] | None:
     return {"model_call_count": state.get("model_call_count", 0) + 1}
 
+
 agent = create_agent(
     model="gpt-4o",
     middleware=[check_call_limit, increment_counter],
-    tools=[...],
+    tools=[],
 )
 
 # Invoke with custom state
@@ -412,14 +418,18 @@ result = agent.invoke({
 <Tab title="Class">
 
 ```python
+from langchain.agents import create_agent
+from langchain.messages import HumanMessage
 from langchain.agents.middleware import AgentState, AgentMiddleware
 from typing_extensions import NotRequired
 from typing import Any
 
+
 class CustomState(AgentState):
     model_call_count: NotRequired[int]
     user_id: NotRequired[str]
 
+
 class CallCounterMiddleware(AgentMiddleware[CustomState]):
     state_schema = CustomState
 
@@ -432,10 +442,11 @@ class CallCounterMiddleware(AgentMiddleware[CustomState]):
     def after_model(self, state: CustomState, runtime) -> dict[str, Any] | None:
         return {"model_call_count": state.get("model_call_count", 0) + 1}
 
+
 agent = create_agent(
     model="gpt-4o",
     middleware=[CallCounterMiddleware()],
-    tools=[...],
+    tools=[],
 )
 
 # Invoke with custom state