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/auth/auth_page_test.dart

@@ -45,7 +45,7 @@ void main() {
     await tester.pumpAndSettle();
   }
 
-  testWidgets('should show the email error and stay on the page when the form is empty', (tester) async {
+  testWidgets('should_show_email_error_and_stay_on_page_when_form_is_empty', (tester) async {
     await pumpAuthPage(tester);
 
     expect(find.text('Welcome back'), findsOneWidget);
@@ -57,7 +57,7 @@ void main() {
     expect(find.text('Welcome back'), findsOneWidget);
   });
 
-  testWidgets('should reject a weak password locally when only letters are typed', (tester) async {
+  testWidgets('should_reject_weak_password_locally_when_only_letters_are_typed', (tester) async {
     await pumpAuthPage(tester);
 
     await tester.enterText(find.byType(TextField).first, 'a@b.com');
@@ -68,7 +68,7 @@ void main() {
     expect(find.text('Password needs both letters and numbers'), findsOneWidget);
   });
 
-  testWidgets('should flip between sign in and sign up when the switch link is tapped', (tester) async {
+  testWidgets('should_flip_between_sign_in_and_sign_up_when_switch_link_is_tapped', (tester) async {
     await pumpAuthPage(tester);
     expect(find.text('Welcome back'), findsOneWidget);