fix(docs): add checkpointer=InMemorySaver() to trim_messages example for persistent memory

[hefeng6500]

Overview

This PR updates the trim_messages example in the documentation to include
checkpointer=InMemorySaver().

Without the checkpointer, each agent.invoke() call starts a new conversation thread, causing the agent to forget previous context (e.g., the user’s name).
This change ensures persistent memory across invocations and aligns the example’s behavior with the printed output shown in the docs.

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Type of change

Type: Update existing documentation

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Related issues/PRs

None directly.
This PR addresses confusion reported by users when reproducing the trim_messages example —
the code output did not match the example’s described behavior without adding a checkpointer.

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Checklist

• I have read the contributing guidelines
• I have tested the code example locally
• The example now behaves as described (Your name is Bob is remembered)
• All code examples work correctly
• Root-relative paths are used for internal links

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Additional notes

This change makes the example self-contained and beginner-friendly.
It clarifies that persistent memory requires an explicit checkpointer when using create_agent() with multi-turn interactions.