agent: Add more Rust code examples, update TODO check (#28737)

Release Notes:

- N/A

crates/eval/examples/email_verification_refactor/prompt.md

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+I want to refactor the `register_verification_email` function to streamline how verification emails are handled. Currently, the code checks if a user exists and then sends an email or returns early. I’d like to move the JWT token generation to the top of the function to avoid duplication. Then, if mail sending is enabled, the code should check for the user, but only send the verification email if the user exists and has a `private_key` (otherwise it should send the email). Keep the random sleep logic for timing mitigation in the branch where no email is sent. Remove the old duplicated token generation logic and any redundant conditionals, while ensuring the core behavior and response flow stays the same.