Use error chain iterators

When traversing error chains in Rust, prefer using the chain() iterator method over manual source traversal with while loops. The chain() method provides a more idiomatic and readable approach to walking through error chains, eliminating the need for manual loop management and making the code more concise.

Instead of manually iterating through error sources:

let mut source = error.source();
while let Some(err) = source {
    if let Some(rl) = err.downcast_ref::<RateLimitedError>() {
        return rl.retry_after;
    }
    source = err.source();
}

Use the iterator-based approach:

error
    .chain()
    .find_map(|e| e.downcast_ref::<RateLimitedError>())
    .and_then(|rl| rl.retry_after)

This pattern works well with other iterator methods like any(), find(), and filter_map() to create more expressive and maintainable error handling code.