Adds an example of a good "general case" SQL tool:
* enforces read-only mode
* hints to the LLM to discover the tables and schemas for the tables it
needs before any query
* uses RetryableToolErrors to hint to the LLM about what to do next
Docs: https://github.com/ArcadeAI/docs/pull/345
For testing, `TEST_POSTGRES_DATABASE_CONNECTION_STRING` has been set in
the repo (from Neon). details in 1 password.
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