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/playback/watch_history_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('resume position clamps invalid and completed values', () {
+  test('should_clamp_invalid_and_completed_values_when_resolving_resume_position', () {
     const duration = Duration(seconds: 8);
     expect(
       clampResumePosition(const Duration(seconds: -1), duration),
@@ -25,7 +25,7 @@ void main() {
     );
   });
 
-  test('watch history is scoped by profile, drama and episode', () async {
+  test('should_scope_history_by_profile_drama_and_episode_when_saving', () async {
     SharedPreferencesAsyncPlatform.instance =
         InMemorySharedPreferencesAsync.empty();
     final history = SharedPreferencesWatchHistoryRepository();
@@ -53,7 +53,7 @@ void main() {
     expect(await history.lastEpisodeId('profile-a', 'drama-b'), isNull);
   });
 
-  test('resume prefers the first loaded episode', () async {
+  test('should_prefer_first_loaded_episode_when_resuming', () async {
     SharedPreferencesAsyncPlatform.instance =
         InMemorySharedPreferencesAsync.empty();
     final history = SharedPreferencesWatchHistoryRepository();
@@ -87,7 +87,7 @@ void main() {
     );
   });
 
-  test('resume falls back to the last watched episode for a list card', () async {
+  test('should_fall_back_to_last_watched_episode_when_resuming_a_list_card', () async {
     SharedPreferencesAsyncPlatform.instance =
         InMemorySharedPreferencesAsync.empty();
     final history = SharedPreferencesWatchHistoryRepository();
@@ -112,7 +112,7 @@ void main() {
     );
   });
 
-  test('a list card with no history has no resume position', () async {
+  test('should_have_no_resume_position_when_list_card_has_no_history', () async {
     SharedPreferencesAsyncPlatform.instance =
         InMemorySharedPreferencesAsync.empty();
     final history = SharedPreferencesWatchHistoryRepository();