fix: AI 复习变式请求字段与校验一致；转写后删除不保留的录音

- generateReviewVariant 的指令要求 stimulus/answer/acceptedAnswers 等字段，
  而 decodeGeneratedReviewVariant 只接受 schemaVersion、variantId、
  targetItemId、prompt、expectedAnswer 五个字段，导致 AI 变式始终校验失败。
  现在指令只要求这五个字段，并补充了请求与解码一致性的测试。
- finishVoiceInput(keepAudio: false) 转写完成（或页面已关闭）后删除音频，
  补练页和测评页的语音输入不再在本机残留录音文件。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kouyu_english/test/generated_content_test.dart

@@ -1,5 +1,11 @@
+import 'dart:convert';
+
 import 'package:flutter_test/flutter_test.dart';
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+import 'package:kouyu_english/core/ai_service.dart';
 import 'package:kouyu_english/core/generated_content.dart';
+import 'package:kouyu_english/core/models.dart';
 
 void main() {
   const valid = '''{
@@ -35,6 +41,54 @@ void main() {
     );
   });
 
+  test(
+    'review variant request asks for exactly the fields it accepts',
+    () async {
+      AiService.instance.setFallbackApiKey('test-key');
+      final prompts = <String>[];
+      final variant = await http.runWithClient(
+        () => AiService.instance.generateReviewVariant(
+          provider: AiProviderType.openAi,
+          endpoint: 'https://example.test/v1',
+          model: 'test-model',
+          targetItemId: 'A0-P12',
+          basePrompt: '请用英语说你来自哪里。',
+        ),
+        () => MockClient((request) async {
+          final body = jsonDecode(request.body) as Map<String, dynamic>;
+          final messages = body['messages'] as List<dynamic>;
+          prompts.add((messages.single as Map<String, dynamic>)['content']);
+          return http.Response(
+            jsonEncode({
+              'choices': [
+                {
+                  'message': {'content': valid},
+                },
+              ],
+            }),
+            200,
+            headers: {'content-type': 'application/json; charset=utf-8'},
+          );
+        }),
+      );
+
+      expect(variant, isNotNull);
+      expect(prompts, hasLength(1));
+      for (final field in [
+        'schemaVersion',
+        'variantId',
+        'targetItemId',
+        'prompt',
+        'expectedAnswer',
+      ]) {
+        expect(prompts.single, contains(field));
+      }
+      for (final field in ['stimulus', 'acceptedAnswers', 'forbiddenPhrases']) {
+        expect(prompts.single, isNot(contains(field)));
+      }
+    },
+  );
+
   const writing = '''{
     "schemaVersion":"writing-feedback-1",
     "lessonId":"a0-06",