newsletter::newsletter_creation_is_idempotent at the end of 11.8 does not actually work

The test requires that the repeated response have a flash message, which is not in fact in the code; I had to add it like so:

```
// Return early if we have a saved response in the database
if let Some(saved_response) = get_saved_response(&pool, &idempotency_key, *user_id)
.await
.map_err(e500)?
{
FlashMessage::info("The newsletter issue has been published!").send();
return Ok(saved_response);
}
```

I believe this is the same method that https://github.com/LukeMathWalker/zero-to-production/blob/root-chapter-11/src/routes/admin/newsletter/post.rs#L50 uses later in the chapter.

Start with the newsletter::newsletter_creation_is_idempotent test at the end of chapter 11.8 and compare it with root-chapter-11/src/routes/admin/newsletter/post.rs around line 50. Verify the repeated request behavior and confirm that the test and implementation agree about the flash message and saved response.

Written by the indexing model from the issue text.