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/playback_session_test.dart

@@ -17,7 +17,7 @@ void main() {
     ),
   );
 
-  test('locked episode never creates or initializes a player', () async {
+  test('should_create_no_player_when_episode_is_locked', () async {
     final factory = FakeFactory();
     final session = PlaybackSession(
       episodes: episodes,
@@ -30,7 +30,7 @@ void main() {
     await session.close();
   });
 
-  test('rapid selection drops stale initialize and keeps one player', () async {
+  test('should_drop_stale_initialize_and_keep_one_player_when_selection_is_rapid', () async {
     final factory = FakeFactory(holdFirstInitialize: true);
     final session = PlaybackSession(
       episodes: episodes,
@@ -52,7 +52,7 @@ void main() {
   });
 
   test(
-    'background and overlay pause without forgetting playback intent',
+    'should_pause_without_forgetting_intent_when_backgrounding_or_overlay_shows',
     () async {
       final factory = FakeFactory();
       final session = PlaybackSession(
@@ -80,7 +80,7 @@ void main() {
     },
   );
 
-  test('resume preparation finishes before video starts', () async {
+  test('should_finish_resume_preparation_before_video_starts_when_resuming', () async {
     final factory = FakeFactory();
     final prepared = Completer<void>();
     final session = PlaybackSession(
@@ -99,7 +99,7 @@ void main() {
     await session.close();
   });
 
-  test('pausing then swiping to the next episode resumes playback', () async {
+  test('should_resume_playback_when_swiping_after_pause', () async {
     final factory = FakeFactory();
     final session = PlaybackSession(
       episodes: episodes,
@@ -121,7 +121,7 @@ void main() {
     await session.close();
   });
 
-  test('re-selecting the same episode keeps the user pause', () async {
+  test('should_keep_user_pause_when_reselecting_the_same_episode', () async {
     final factory = FakeFactory();
     final session = PlaybackSession(
       episodes: episodes,
@@ -140,7 +140,7 @@ void main() {
     await session.close();
   });
 
-  test('automatic advance respects visibility and playback intent', () async {
+  test('should_respect_visibility_and_intent_when_advancing_automatically', () async {
     final factory = FakeFactory();
     final session = PlaybackSession(
       episodes: episodes,