MM-37787: ThreadAutoFollow must be true to enable CollapsedThreads (#18253)

* MM-37787: ThreadAutoFollow must be true to enable CollapsedThreads

* check if defaults are valid

model/config_test.go

@@ -1467,3 +1467,29 @@ func TestConfigExportSettingsIsValid(t *testing.T) {
 	require.NotNil(t, err)
 	require.Equal(t, "model.config.is_valid.export.retention_days_too_low.app_error", err.Id)
 }
+
+func TestConfigServiceSettingsIsValid(t *testing.T) {
+	cfg := Config{}
+	cfg.SetDefaults()
+
+	err := cfg.ServiceSettings.isValid()
+	require.Nil(t, err)
+
+	*cfg.ServiceSettings.CollapsedThreads = CollapsedThreadsDisabled
+	err = cfg.ServiceSettings.isValid()
+	require.Nil(t, err)
+
+	*cfg.ServiceSettings.ThreadAutoFollow = false
+	err = cfg.ServiceSettings.isValid()
+	require.Nil(t, err)
+
+	*cfg.ServiceSettings.CollapsedThreads = CollapsedThreadsDefaultOff
+	err = cfg.ServiceSettings.isValid()
+	require.NotNil(t, err)
+	require.Equal(t, "model.config.is_valid.collapsed_threads.autofollow.app_error", err.Id)
+
+	*cfg.ServiceSettings.CollapsedThreads = CollapsedThreadsDefaultOn
+	err = cfg.ServiceSettings.isValid()
+	require.NotNil(t, err)
+	require.Equal(t, "model.config.is_valid.collapsed_threads.autofollow.app_error", err.Id)
+}