feat(examples): Add sql_agent example

[pokey]

To accompany https://docs.langchain.com/oss/javascript/langgraph/sql-agent from langchain-ai/docs#1050

• Depends on feat(core): Add BaseMessage.toFormattedString() #9228

I'm not sure this is in the right place, but easy to move

[changeset-bot]

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[pokey]

superseded by langchain-ai/langgraphjs#1750