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

core/lib/presentation/views/dialog/modal_list_action_button_widget.dart

@@ -55,14 +55,17 @@ class ModalListActionButtonWidget extends StatelessWidget {
       ),
     );
 
-    Widget progressingButton = const SizedBox(
+    Widget progressingButton = SizedBox(
       height: 48,
       width: 153,
       child: ConfirmDialogButton(
-        label: '',
+        label: positiveLabel,
         padding: EdgeInsets.zero,
         backgroundColor: AppColor.primaryMain,
-        child: CupertinoLoadingWidget(color: Colors.white),
+        child: CupertinoLoadingWidget(
+          color: Colors.white,
+          semanticLabel: positiveLabel,
+        ),
       ),
     );