MM-46947 - Fix: Manifest parsing won't allow placeholders for custom pluginSettingType (#21004)

* allow custom pluginSettingTypes to have a placeholder

* i18n

* lock it down with a test case

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/manifest.go

@@ -364,8 +364,9 @@ func (s *PluginSetting) isValid() error {
 		pluginSettingType == Text ||
 		pluginSettingType == LongText ||
 		pluginSettingType == Number ||
-		pluginSettingType == Username) {
-		return errors.New("should not set Placeholder for setting type not in text, generated or username")
+		pluginSettingType == Username ||
+		pluginSettingType == Custom) {
+		return errors.New("should not set Placeholder for setting type not in text, generated, number, username, or custom")
 	}
 
 	if s.Options != nil {

model/manifest_test.go

@@ -184,6 +184,13 @@ func TestSettingIsValid(t *testing.T) {
 			},
 			false,
 		},
+		"Placeholder is allowed for custom settings": {
+			PluginSetting{
+				Type:        "custom",
+				Placeholder: "some Text",
+			},
+			false,
+		},
 	} {
 		t.Run(name, func(t *testing.T) {
 			err := test.Setting.isValid()