fix(pam/gdmmodel): Forward public AuthModeSelected to GDM (#631)

authModeSelected event we were listening to is an event that may be
ignored or adjusted depending on authModeSelection model logic, so it's
what we should also expose to GDM.

We couldn't do that though since the event wasn't forwarded, so do it.

Also simplify user selection on GDM tests, by mimicking more what we
really do.

It should help with flacky tests such as [1].

[1]
https://github.com/ubuntu/authd/actions/runs/11802896113/job/32879602555?pr=583

pam/internal/adapter/gdmmodel.go

@@ -107,9 +107,7 @@ func (m *gdmModel) pollGdm() tea.Cmd {
 	for _, result := range gdmPollResults {
 		switch res := result.Data.(type) {
 		case *gdm.EventData_UserSelected:
-			commands = append(commands, sendEvent(userSelected{
-				username: res.UserSelected.UserId,
-			}))
+			commands = append(commands, sendUserSelected(res.UserSelected.UserId))
 
 		case *gdm.EventData_BrokerSelected:
 			if res.BrokerSelected == nil {
@@ -217,9 +215,9 @@ func (m gdmModel) Update(msg tea.Msg) (gdmModel, tea.Cmd) {
 			AuthModesReceived: &gdm.Events_AuthModesReceived{AuthModes: msg.authModes},
 		})
 
-	case authModeSelected:
+	case AuthModeSelected:
 		return m, m.emitEvent(&gdm.EventData_AuthModeSelected{
-			AuthModeSelected: &gdm.Events_AuthModeSelected{AuthModeId: msg.id},
+			AuthModeSelected: &gdm.Events_AuthModeSelected{AuthModeId: msg.ID},
 		})
 
 	case UILayoutReceived: