TF-4301 Address code review comments on label feature

- Make LabelModalMixin interactor getters non-nullable; remove force-unwraps at call sites
- Show positiveLabel and add semanticLabel in progressing button for accessibility
- Use timestamp-based label name in E2E scenario to prevent flakiness on reused accounts
- Guard labelAs action with isLabelAvailable check in SingleEmailController
- Don't pop back on label creation failure so users can retry without losing context
- Block modal dismissal while label creation request is in flight
- Null-guard email.id before toggling label in ThreadView::onCreateANewLabelAction
- Log stackTrace and rethrow in DialogRouter to distinguish errors from user cancel

lib/features/labels/presentation/widgets/create_new_label_modal.dart

@@ -422,6 +422,9 @@ class _CreateNewLabelModalState extends State<CreateNewLabelModal> {
   }
 
   void _onCloseModal() {
+    if (_createLabelStateNotifier.value == LabelPositiveButtonState.progressing) {
+      return;
+    }
     _clearInputFocus();
     popBack();
   }