docs(utils): make the retry_with_backoff_if doctest actually compile

The example referenced an unimported DbError whose type didn't even
match the closure's anyhow::Error, and the classifier body returned
(). Rewritten as a self-contained io::Error example with an assertion.

src/utils.rs

@@ -63,11 +63,15 @@ where
 /// ```
 /// use image_api::utils::retry_with_backoff_if;
 ///
-/// fn is_transient(e: &DbError) -> bool { /* ... */ }
+/// fn is_transient(e: &std::io::Error) -> bool {
+///     // Only retry connection-class failures; a NotFound is deterministic.
+///     e.kind() == std::io::ErrorKind::ConnectionRefused
+/// }
 ///
 /// let result = retry_with_backoff_if("some-write", 4, is_transient, || {
-///     Ok::<_, anyhow::Error>(42)
+///     Ok::<i32, _>(42)
 /// });
+/// assert!(result.is_ok_and(|v| v == 42));
 /// ```
 pub fn retry_with_backoff_if<F, T, E, U: Fn(&E) -> bool>(
     label: &str,