Error loop with custom domain validation using external DNS

### Environment information

```plain text
Not Applicable / Using AMP1
```

### Description

### Steps to reproduce
1. Set up custom domain
2. Start the process / generate the SSL cert
3. Enter the CNAME into the DNS server after 1 day (simulate propagation time)
4. Go back Amplify control panel. DNS authorization failed. Retry button is the only option,
5. Click the retry button
6. A new SSL certificate is generated
7. A new CNAME key is generated
8. The validation fails because Amplify sees the old CNAME in the DNS
9. Go back to step 4.

No source file or test is named. Start with the custom-domain setup and SSL certificate generation flow in the Amplify control panel, then reproduce the delayed DNS propagation sequence. Done means retrying validation after propagation no longer loops by generating a new CNAME that invalidates the existing DNS record.

Written by the indexing model from the issue text.