fix(audience-sample): inject tick label into test-mode and mobile-attribution toggles

Same Unity 2021.3 runtime-panel workaround already applied to the debug
toggle: without the injected Label the checked state renders as a plain
coloured square with no visible tick.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nattb8

examples/audience/Assets/SampleApp/Scripts/AudienceSample.UI.cs

@@ -187,13 +187,14 @@ private void BindElements()
             // Inject a tick Label — Unity 2021.3 runtime panels render the
             // checked state as a plain coloured square otherwise. USS hides
             // the tick when unchecked.
-            var debugCheckmark = _debug.Q<VisualElement>(className: "unity-toggle__checkmark");
-            if (debugCheckmark != null)
+            foreach (var toggle in new[] { _debug, _testMode, _enableMobileAttribution })
             {
+                var checkmark = toggle.Q<VisualElement>(className: "unity-toggle__checkmark");
+                if (checkmark == null) continue;
                 var tick = new Label("✓");
                 tick.AddToClassList("debug-tick");
                 tick.pickingMode = PickingMode.Ignore;
-                debugCheckmark.Add(tick);
+                checkmark.Add(tick);
             }
             _flushInterval = Require<TextField>("flush-interval");
             _flushSize     = Require<TextField>("flush-size");