test(client): 用例名统一为 should_<期望>_when_<条件>（155/155）

为什么：红线第 8 条门禁立即生效，要求 Flutter 测试命名合规；此前 155 条用例名是英文自然语言
或中文描述，合规数 0/155，属存量欠账，按"先补齐再上新功能"在本批处理。

- 25 个测试文件、155 条用例改名，按「文件 + 旧名」精确定位，每条断言命中恰好 1 次才替换
- 只改名字字面量：改动行 155 删 + 155 增，无一行不含字符串字面量（断言与用例体零改动）
- 门禁：dart analyze lib test = No issues found；flutter test = 155 全过

test/engagement/engagement_controller_test.dart

@@ -5,7 +5,7 @@ import 'package:shared_preferences_platform_interface/in_memory_shared_preferenc
 import 'package:shared_preferences_platform_interface/shared_preferences_async_platform_interface.dart';
 
 void main() {
-  test('saved, liked and comments are kept by drama id', () async {
+  test('should_keep_saved_liked_and_comments_by_drama_id_when_toggled', () async {
     final repository = InMemoryEngagementRepository();
     final first = EngagementController(repository);
     await first.load();
@@ -28,7 +28,7 @@ void main() {
     expect(reopened.isSaved('signal'), isFalse);
   });
 
-  test('local engagement survives a new repository instance', () async {
+  test('should_survive_new_repository_instance_when_engagement_is_local', () async {
     SharedPreferencesAsyncPlatform.instance =
         InMemorySharedPreferencesAsync.empty();
     final first = EngagementController(SharedPreferencesEngagementRepository());

test/engagement/engagement_repository_test.dart

@@ -27,7 +27,7 @@ void main() {
     return repository().load();
   }
 
-  test('a first launch reads an empty snapshot', () async {
+  test('should_read_empty_snapshot_when_first_launch', () async {
     final snapshot = await repository().load();
 
     expect(snapshot.savedDramaIds, isEmpty);
@@ -35,7 +35,7 @@ void main() {
     expect(snapshot.commentsByDrama, isEmpty);
   });
 
-  test('a snapshot survives a save and reload round trip', () async {
+  test('should_survive_save_and_reload_round_trip_when_snapshot_is_stored', () async {
     await repository().save(
       const EngagementSnapshot(
         savedDramaIds: {'signal', 'city'},
@@ -55,7 +55,7 @@ void main() {
     });
   });
 
-  test('an empty snapshot round trips as empty collections', () async {
+  test('should_round_trip_as_empty_collections_when_snapshot_is_empty', () async {
     await repository().save(const EngagementSnapshot());
 
     final loaded = await repository().load();
@@ -65,7 +65,7 @@ void main() {
     expect(loaded.commentsByDrama, isEmpty);
   });
 
-  test('corrupt or non-object payloads fall back to an empty snapshot', () async {
+  test('should_fall_back_to_empty_snapshot_when_payload_is_corrupt_or_not_an_object', () async {
     for (final raw in <String>[
       '',
       'not json at all',
@@ -82,7 +82,7 @@ void main() {
     }
   });
 
-  test('wrongly typed fields are dropped instead of throwing', () async {
+  test('should_drop_mistyped_fields_when_payload_has_wrong_types', () async {
     final snapshot = await loadRaw(
       jsonEncode({
         'saved': 'signal',
@@ -103,7 +103,7 @@ void main() {
     });
   });
 
-  test('a payload missing keys falls back per field', () async {
+  test('should_fall_back_per_field_when_payload_keys_are_missing', () async {
     final snapshot = await loadRaw(jsonEncode({'saved': ['signal']}));
 
     expect(snapshot.savedDramaIds, {'signal'});
@@ -111,7 +111,7 @@ void main() {
     expect(snapshot.commentsByDrama, isEmpty);
   });
 
-  test('save writes only json primitives', () async {
+  test('should_write_only_json_primitives_when_saving', () async {
     await repository().save(
       const EngagementSnapshot(
         savedDramaIds: {'signal'},