FR: Webhook test endpoint on demand

### What are you trying to do?

I'm writing a webhook handler, and trying to test it using the Test endpoint feature. If my handler fails to process the test event it is resent every hour, and it is not possible to manually resend it. Also there is no UI to show that sending a test event is now failing.

### How should we solve this?

I think the test event should not be handled like other webhooks. If it fails maybe it should not be resent. You should immediately be able to request a new test event even if the last one failed.

### What is the impact of not solving this?

If a webhook handler is failing, you can only re-test it once per hour.

### Anything else?

_No response_

Start by tracing the Test endpoint feature and its handling of failed webhook test events; the issue does not identify files or tests. Done means a failed test event can be requested again immediately, is not automatically resent hourly, and the UI indicates when test delivery fails.

Written by the indexing model from the issue text.