pam/integration-tests/gdm: Try doing broker selection also on receive…

…d brokers

We have a race on broker selection because we may choose a broker when
we don't have one yet, as this depends on the order of the events that
may not always be the same.

To prevent this:
 1. If received the brokers and we're in broker selection stage
    -> Do the broker selection right away, marking it as done
 2. If we just switched to broker selection stage
    a. If the brokers have been received, try to switch to the selected
       one or error if not found.
    b. If brokers have not yet been received, we just ignore the request
       and perform it once the brokers have been provided.

Partially go back to 48db852 logic.

See: https://github.com/ubuntu/authd/actions/runs/12144656077/job/33864519830

pam/integration-tests/gdm-module-handler_test.go

@@ -122,6 +122,22 @@ func (gh *gdmTestModuleHandler) exampleHandleEvent(event *gdm.EventData) error {
 		}
 		gh.brokersInfos = ev.BrokersReceived.BrokersInfos
 
+		if gh.currentStage != proto.Stage_brokerSelection {
+			return nil
+		}
+		if gh.selectedBrokerName == ignoredBrokerName {
+			return nil
+		}
+		idx := slices.IndexFunc(gh.brokersInfos, func(bi *authd.ABResponse_BrokerInfo) bool {
+			return bi.Name == gh.selectedBrokerName
+		})
+		if idx < 0 {
+			return fmt.Errorf("broker %q is not known", gh.selectedBrokerName)
+		}
+
+		gh.pollResponses = append(gh.pollResponses,
+			gdm_test.SelectBrokerEvent(gh.brokersInfos[idx].Id))
+
 	case *gdm.EventData_BrokerSelected:
 		idx := slices.IndexFunc(gh.brokersInfos, func(broker *authd.ABResponse_BrokerInfo) bool {
 			return broker.Id == ev.BrokerSelected.BrokerId
@@ -219,11 +235,15 @@ func (gh *gdmTestModuleHandler) exampleHandleAuthDRequest(gdmData *gdm.Data) (*g
 			idx := slices.IndexFunc(gh.brokersInfos, func(bi *authd.ABResponse_BrokerInfo) bool {
 				return bi.Name == gh.selectedBrokerName
 			})
-			if idx < 0 {
+			if idx < 0 && len(gh.brokersInfos) > 0 {
 				return nil, fmt.Errorf("broker '%s' is not known", gh.selectedBrokerName)
 			}
+			if idx < 0 {
+				break
+			}
 
 			gh.pollResponses = append(gh.pollResponses, gdm_test.SelectBrokerEvent(gh.brokersInfos[idx].Id))
+			gh.selectedBrokerName = ignoredBrokerName
 
 		case proto.Stage_authModeSelection:
 			gh.currentUILayout = nil