fix(mobile-app): 合同签署页面添加重试机制解决竞态问题

前端在支付完成后立即请求合同签署任务，但后端 Kafka 事件可能还未完成
任务创建。添加最多5次重试，使用指数退避策略（500ms * attempt）。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/contract_signing/presentation/pages/contract_signing_page.dart

@@ -81,37 +81,55 @@ class _ContractSigningPageState extends ConsumerState<ContractSigningPage> {
     super.dispose();
   }
 
-  /// 加载签署任务
+  /// 加载签署任务（带重试机制，解决竞态问题）
   Future<void> _loadTask() async {
     setState(() {
       _isLoading = true;
       _errorMessage = null;
     });
 
-    try {
-      final service = ref.read(contractSigningServiceProvider);
-      final task = await service.getTask(widget.orderNo);
+    const maxRetries = 5;
+    const retryDelay = Duration(milliseconds: 500);
 
-      setState(() {
-        _task = task;
-        _isLoading = false;
-        _remainingSeconds = task.remainingSeconds;
-        _hasScrolledToBottom = task.scrolledToBottomAt != null;
-        _hasAcknowledged = task.acknowledgedAt != null;
-      });
+    for (int attempt = 1; attempt <= maxRetries; attempt++) {
+      try {
+        final service = ref.read(contractSigningServiceProvider);
+        final task = await service.getTask(widget.orderNo);
 
-      // 启动倒计时
-      _startCountdown();
+        setState(() {
+          _task = task;
+          _isLoading = false;
+          _remainingSeconds = task.remainingSeconds;
+          _hasScrolledToBottom = task.scrolledToBottomAt != null;
+          _hasAcknowledged = task.acknowledgedAt != null;
+        });
 
-      // 加载 PDF
-      if (task.status != ContractSigningStatus.signed) {
-        _loadPdf();
+        // 启动倒计时
+        _startCountdown();
+
+        // 加载 PDF
+        if (task.status != ContractSigningStatus.signed) {
+          _loadPdf();
+        }
+        return; // 成功，退出重试循环
+      } catch (e) {
+        debugPrint('[ContractSigningPage] 加载任务失败 (尝试 $attempt/$maxRetries): $e');
+
+        // 如果是"任务不存在"错误且不是最后一次尝试，则等待后重试
+        // 这是为了处理任务创建的竞态问题
+        if (attempt < maxRetries && e.toString().contains('不存在')) {
+          debugPrint('[ContractSigningPage] 等待 ${retryDelay.inMilliseconds}ms 后重试...');
+          await Future.delayed(retryDelay * attempt); // 指数退避
+          continue;
+        }
+
+        // 最后一次尝试或其他错误
+        setState(() {
+          _isLoading = false;
+          _errorMessage = '加载签署任务失败: $e';
+        });
+        return;
       }
-    } catch (e) {
-      setState(() {
-        _isLoading = false;
-        _errorMessage = '加载签署任务失败: $e';
-      });
     }
   }