Please address the comments from this code review:

## Overall Comments
- In `TestShutdown`, the `suite.Failf` call in the `RetryCommand` select branch passes a format string without placeholders and a separate "rid=%s" string, so the failure message won’t include the `rid` value as intended; update it to something like `suite.Failf("Expected channel to be closed or empty, got value rid=%s", rid)`.
- For the new `UnconfirmedMessageHandlerImpl.Stopped()`/cleanup behavior, consider explicitly documenting the ordering guarantees (e.g., that all timers are stopped and channels are closed before `Stopped()` is reported) to make it clear to future callers how safe it is to perform operations concurrently with shutdown.

## Individual Comments

### Comment 1
<location> `pkg/retry/handler/unconfirmed_message_handler_test.go:181-190` </location>
<code_context>

-	// Non-blocking notify of ACK
+	// Non-blocking notify of ACK (channel may be closed if handler stopped)
 	select {
 	case <-h.ctx.Done():
 		return
</code_context>

<issue_to_address>
**issue (bug_risk):** Fix `Failf` formatting so the resource ID is actually included in the failure message

`Failf` uses its first argument as the format string, so in your current code the `
</issue_to_address>