重构ClipboardRecordingModel以支持可选的时间戳，并更新相关测试以改进JSON解析和验证。

lib/features/recording/model/model_clipboard.dart

@@ -19,8 +19,8 @@ class ClipboardRecordingModel {
   factory ClipboardRecordingModel.fromJson(Map<String, dynamic> json) {
     return ClipboardRecordingModel(
       title: _readString(json, 'title'),
-      startTimestamp: _readInt(json, 'startTimestamp'),
-      endTimestamp: _readInt(json, 'endTimestamp'),
+      startTimestamp: _readOptionalInt(json, 'startTimestamp'),
+      endTimestamp: _readOptionalInt(json, 'endTimestamp'),
       address: _readString(json, 'address'),
       filename: _readOptionalString(json, 'filename'),
     );
@@ -52,8 +52,9 @@ class ClipboardRecordingModel {
     throw FormatException('Clipboard field "$key" must be a String.');
   }
 
-  static int _readInt(Map<String, dynamic> json, String key) {
+  static int? _readOptionalInt(Map<String, dynamic> json, String key) {
     final value = json[key];
+    if (value == null) return null;
     if (value is int) return value;
     throw FormatException('Clipboard field "$key" must be an int.');
   }