Fix school browser initialization and redirect detection; improve diagnostics

mobile/test/features/feedback_http_sender_test.dart

@@ -33,6 +33,39 @@ class FakeFeedbackHttpTransport implements FeedbackHttpTransport {
 }
 
 void main() {
+  test(
+    'old feedback service gets a specific diagnostic-upgrade hint',
+    () async {
+      final transport = FakeFeedbackHttpTransport()
+        ..response = FeedbackHttpResponse(
+          422,
+          utf8.encode(
+            '{"error":"validation_error","detail":"Unexpected file field."}',
+          ),
+        );
+      await expectLater(
+        HttpFeedbackSender(
+          endpoint: 'https://example.com/v1/feedback',
+          transport: transport,
+        ).send(
+          FeedbackSubmission.problem(
+            id: 'same-id',
+            message: '问题',
+            diagnosticLog: Uint8List.fromList(
+              utf8.encode('TPSchedule diagnostic log\n'),
+            ),
+          ),
+        ),
+        throwsA(
+          isA<FeedbackException>().having(
+            (e) => e.message,
+            'upgrade hint',
+            allOf(contains('日志'), contains('更新')),
+          ),
+        ),
+      );
+    },
+  );
   test('diagnostics use a separate UTF8 attachment field', () async {
     final transport = FakeFeedbackHttpTransport();
     await HttpFeedbackSender(