Fix returned pagination param for alert_class_list (#8678)

Currently the `alert_receiver_list` endpoint will return a String as
its pagination parameter, serialized as
`{"v":"v1","page_start":"probe"}`. However, this endpoint is expecting a
`AlertClassPage` struct in its query parameters, causing clients that
use the returned pagination token in their subsequent request to fail
with error "failed to parse pagination token: corrupted token".

To resolve this, set the page selector to return the expected
`AlertClassPage` instead.

Closes #8660

Author: wfchandler

nexus/src/external_api/http_entrypoints.rs

@@ -8192,7 +8192,9 @@ impl NexusExternalApi for NexusExternalApiImpl {
             Ok(HttpResponseOk(ResultsPage::new(
                 alert_classes,
                 &EmptyScanParams {},
-                |class: &views::AlertClass, _| class.name.clone(),
+                |class: &views::AlertClass, _| params::AlertClassPage {
+                    last_seen: class.name.clone(),
+                },
             )?))
         };
         apictx