UI Bug in provisioned opsgenie alerts

On Provisioned Contact Points for OpsGenie, the UI appears to have a bug where it is requiring the OpsGenie URL be filled in, even though it is defaulting to the correct URL on the backend (we did not provide a URL in our Terraform config for the contact point, relying on the default).

It appears to only surface when you are using the Test button.

Steps to reproduce:

1. Provision an OpsGenie Contact Point without the Alert URL set through the API
2. Click Test, run a test
3. Close the test frame
4. Now you can no longer click test as it is erroring out on the missing alert URL

Looks like the default is being set here: https://github.com/grafana/alerting/blob/2dda1c67ec02625ac9fc8607157b3d5825d47919/receivers/opsgenie/config.go#L64-L66

The UI should fill this in on the user facing side as well.

Start with receivers/opsgenie/config.go at the linked default Alert URL handling, then trace the provisioned contact-point UI and its Test flow. Reproduce the issue by omitting the URL, running Test, closing the frame, and testing again. Done means the UI retains or displays the backend default so repeated tests no longer fail on a missing Alert URL.

Written by the indexing model from the issue text.