Resolves https://github.com/langchain-ai/langchain/issues/30724
The [prompt in
langchain-hub](https://smith.langchain.com/hub/langchain-ai/sql-query-system-prompt)
used in this guide was composed of just a system message, but the guide
did not add a human message to it. This was incompatible with some
providers (and is generally not a typical usage pattern).
The prompt in prompt hub has been updated to split the question into a
separate HumanMessage. Here we update the guide to reflect this.
ccurme