pam/adapter/userSelection: Do not expose user name until it has been …

…validated

If an user is quick enough to type an user name on the user selection
field we may end up passing such (potentially partial) value to the
broker as the selected user, breaking the rest of the transaction.

This happened because we returned the text input Value as is, without
checking if the user ever ack'ed it via an enter press.

What may happen in short is:
 - adapter.supportedUILayoutsReceived{}
 - adapter.userRequired{}
   * User starts typing something in the user name field
 - adapter.brokersListReceived{brokers:[]{...}}
 - adapter.UsernameOrBrokerListReceived{}
   * User name is set at this point as the form Value() even if not validated
 - adapter.brokerSelectionRequired{}
 - adapter.ChangeStage{Stage:1}

See for example
 - https://github.com/3v1n0/authd/actions/runs/11390824079/job/31693266490

To prevent this, only return a valid username value after the user
selection has been completed during each focus phase that the view has

pam/internal/adapter/model.go

@@ -431,7 +431,7 @@ func (m *UIModel) ExitStatus() PamReturnStatus {
 
 // username returns currently selected user name.
 func (m UIModel) username() string {
-	return m.userSelectionModel.Value()
+	return m.userSelectionModel.Username()
 }
 
 // availableBrokers returns currently available brokers.

pam/internal/adapter/userselection.go

@@ -19,6 +19,7 @@ type userSelectionModel struct {
 	pamMTx     pam.ModuleTransaction
 	clientType PamClientType
 	enabled    bool
+	selected   bool
 }
 
 // userSelected events to report that a new username has been selected.
@@ -91,6 +92,7 @@ func (m userSelectionModel) Update(msg tea.Msg) (userSelectionModel, tea.Cmd) {
 		}
 		if msg.username != "" {
 			// synchronise our internal validated field and the text one.
+			m.selected = true
 			m.SetValue(msg.username)
 			return m, sendEvent(UsernameOrBrokerListReceived{})
 		}
@@ -141,3 +143,19 @@ func (m userSelectionModel) View() string {
 func (m userSelectionModel) Enabled() bool {
 	return m.enabled
 }
+
+// Username returns the approved value of the text input.
+func (m userSelectionModel) Username() string {
+	if m.clientType == InteractiveTerminal && !m.selected {
+		return ""
+	}
+	return m.Model.Value()
+}
+
+// Focus sets the focus state on the model. We also mark as the user is not
+// selected so that the returned value won't be valid until the user did an
+// explicit ack.
+func (m *userSelectionModel) Focus() tea.Cmd {
+	m.selected = false
+	return m.Model.Focus()
+}