tests: Add test for avatar error placeholder.

This adds a new test case to verify the behavior of the AvatarImage widget when the avatar URL fails to load.

The test uses HttpOverrides with a MockHttpClient to reliably simulate a network error. It then asserts that the expected placeholder widget (identified by its icon, ZulipIcons.person) is rendered, ensuring the error handling is working correctly.

Author: vinodsingh-dev-gsoc

test/widgets/user_test.dart

@@ -1,10 +1,13 @@
+import 'dart:io';
+
 import 'package:checks/checks.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/model/store.dart';
 import 'package:zulip/widgets/content.dart';
+import 'package:zulip/widgets/icons.dart';
 import 'package:zulip/widgets/store.dart';
 import 'package:zulip/widgets/user.dart';
 
@@ -13,6 +16,15 @@ import '../model/binding.dart';
 import '../model/test_store.dart';
 import '../stdlib_checks.dart';
 import '../test_images.dart';
+import 'test_app.dart';
+
+class MockHttpClient extends Fake implements HttpClient {
+  @override
+  Future<HttpClientRequest> getUrl(Uri url) async {
+    throw const SocketException('test error');
+  }
+}
+
 
 void main() {
   TestZulipBinding.ensureInitialized();
@@ -73,6 +85,30 @@ void main() {
       debugNetworkImageHttpClientProvider = null;
     });
 
+    testWidgets('shows placeholder when image URL gives error', (WidgetTester tester) async {
+      await HttpOverrides.runZoned(() async {
+        addTearDown(testBinding.reset);
+        await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+        final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+        final user = eg.user(avatarUrl: 'https://zulip.com/avatar.png');
+        await store.addUser(user);
+
+        await tester.pumpWidget(
+          TestZulipApp(
+            accountId: eg.selfAccount.id,
+            child: AvatarImage(userId: user.userId, size: 32),
+          ),
+        );
+        await tester.pump(); // Image provider is created
+        await tester.pump(); // Image fails to load
+        expect(find.byIcon(ZulipIcons.person), findsOneWidget);
+
+        expect(find.byType(DecoratedBox), findsOneWidget);
+
+      }, createHttpClient: (context) => MockHttpClient());
+    });
+
+
     testWidgets('smoke with invalid URL', (tester) async {
       const avatarUrl = '::not a URL::';
       check(await actualUrl(tester, avatarUrl)).isNull();