docs: Add prominent warning about state_schema requirement for custom fields

- Fix TypedDict extension example: remove redefined messages field
- Improve security: replace password field with user_id in example
- Simplify example by removing unnecessary imports
- Fix docstring formatting: remove extra blank lines in Example section

Author: arrdel

libs/langchain_v1/langchain/agents/factory.py

@@ -630,14 +630,11 @@ def create_agent(  # noqa: PLR0915
                 Example:
 
                 ```python
-                from typing import Annotated
                 from langchain.agents import AgentState, create_agent
-                from langgraph.graph import add_messages
 
                 class MyState(AgentState):
-                    messages: Annotated[list, add_messages]
                     authenticated: bool
-                    password: str
+                    user_id: str
 
                 agent = create_agent(
                     model=model,
@@ -692,12 +689,10 @@ class MyState(AgentState):
         ```python
         from langchain.agents import create_agent
 
-
         def check_weather(location: str) -> str:
             '''Return the weather forecast for the specified location.'''
             return f"It's always sunny in {location}"
 
-
         graph = create_agent(
             model="anthropic:claude-sonnet-4-5-20250929",
             tools=[check_weather],