fix(docs): Update example code in Test Docs

[Umjiseung]

Updated agent invocation to use structured messages and configuration

Overview

This PR updates the documentation example for agent.invoke() to correctly demonstrate:

• Passing structured messages using {"messages": [HumanMessage(...)]}
• Passing configuration using
  config={"configurable": {"thread_id": "<session-id>"}}
• Proper usage of checkpoint-based memory with InMemorySaver

Previously, the example omitted or incorrectly used the config argument, which caused confusion and errors when users attempted to maintain session-based memory.
This update provides a fully functional example that aligns with current agent APIs.

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Type of change

Type: Update existing documentation

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Related issues/PRs

None

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Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and confirmed working
• I have used root-relative paths for internal links
• I have updated navigation in src/docs.json if needed

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Additional notes

This update ensures that the documentation demonstrates the correct pattern for invoking agents that use checkpointing or session-based memory. Without a configurable.thread_id, each call creates a new memory session, making it impossible to persist user context.

The updated, working example is shown below:

from langgraph.checkpoint.memory import InMemorySaver
from langchain.agents import create_agent
from langchain_core.messages import HumanMessage

checkpointer = InMemorySaver()

agent = create_agent(
    model="google_genai:gemini-2.5-flash-lite",
    tools=[],
    checkpointer=checkpointer
)

# First invocation
agent.invoke(
    {"messages": [HumanMessage(content="I live in Sydney, Australia")]},
    config={"configurable": {"thread_id": "session-1"}}
)

# Second invocation: the first message is persisted (Sydney location), so the model returns GMT+10 time
agent.invoke(
    {"messages": [HumanMessage(content="What's my local time?")]},
    config={"configurable": {"thread_id": "session-1"}}
)