fix: harden school imports and add optional feedback diagnostics

mobile/test/school/school_session_flow_test.dart

@@ -1,4 +1,7 @@
 import 'dart:async';
+import 'dart:convert';
+
+import 'package:hepan_everyday/services/diagnostic_log.dart';
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:webview_flutter_platform_interface/webview_flutter_platform_interface.dart';
@@ -26,6 +29,30 @@ void main() {
     platform.delegate.finished(url);
   }
 
+  test('login diagnostic breadcrumbs distinguish probe and network failures without raw details', () async {
+    await session.openTimetable(term: SchoolTerm(73));
+    session.setReadingActive(true);
+    platform.web.javascript = (_) async => throw StateError('PRIVATE_TICKET');
+    navigate('https://yjsjy.uestc.edu.cn/pyxx/home/index');
+    await flushSchoolEvents();
+    var log = utf8.decode(await DiagnosticLog.instance.export());
+    expect(log, contains('school_probe_failed'));
+    expect(log, isNot(contains('PRIVATE_TICKET')));
+    platform.delegate.resourceError(
+      WebResourceError(
+        errorCode: -2,
+        description: 'PRIVATE_NETWORK_DETAIL',
+        isForMainFrame: true,
+        url: platform.web.url,
+      ),
+    );
+    await flushSchoolEvents();
+    log = utf8.decode(await DiagnosticLog.instance.export());
+    expect(log, contains('web_resource_error'));
+    expect(log, contains('"status":-2'));
+    expect(log, isNot(contains('PRIVATE_NETWORK_DETAIL')));
+  });
+
   test('graduate logout function automatically opens timetable without an exit anchor', () async {
     await session.openTimetable(term: SchoolTerm(73));
     platform.web.javascript = (script) async =>