Alerting: Fix "Not Implemented" responses (#57710)

* fix swagger spec, return 404 instead of 501 when an endpoint does not exist

* update number of paths in authorization_test.go

pkg/services/ngalert/api/authorization_test.go

@@ -49,7 +49,7 @@ func TestAuthorize(t *testing.T) {
 		}
 		paths[p] = methods
 	}
-	require.Len(t, paths, 41)
+	require.Len(t, paths, 40)
 
 	ac := acmock.New()
 	api := &API{AccessControl: ac}