fix: harden school imports and add optional feedback diagnostics

mobile/test/school/undergraduate_timetable_adapter_test.dart

@@ -9,6 +9,21 @@ import 'package:hepan_everyday/data/schedule_file_import.dart';
 import 'package:hepan_everyday/domain/school_identity.dart';
 
 void main() {
+  test('unrecognized activity writes and incomplete declarations reject the entire timetable', () {
+    const valid =
+        'activity = new TaskActivity("T", "教师", "C", "课程(C.01)", "R", "教室", "010"); '
+        'index = 0*unitCount+0; table0.activities[index][table0.activities[index].length]=activity;';
+    for (final extra in [
+      'table0.activities[11][table0.activities[11].length]=activity;',
+      'activity = new TaskActivity("T", "教师", "C", "另一课程(C.02)", "R", "教室", "010")\n'
+          'index = 0*unitCount+1; table0.activities[index][table0.activities[index].length]=activity;',
+    ]) {
+      expect(
+        () => parseActivityData('$valid $extra', unitCount: 12),
+        throwsFormatException,
+      );
+    }
+  });
   final uri = Uri.parse(
     'https://eams.uestc.edu.cn/eams/courseTableForStd.action',
   );