fix(cli): ignore space in custom input mode (#21752)

Fixes: https://github.com/coder/internal/issues/560

"Select" CLI UI component should ignore "space" when `+Add custom value`
is highlighted. Otherwise it interprets that as a potential option...
and panics.

cli/cliui/select.go

@@ -491,6 +491,11 @@ func (m multiSelectModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 
 		case tea.KeySpace:
 			options := m.filteredOptions()
+
+			if m.enableCustomInput && m.cursor == len(options) {
+				return m, nil
+			}
+
 			if len(options) != 0 {
 				options[m.cursor].chosen = !options[m.cursor].chosen
 			}