fix(auth,web): fail onWaitInitState instead of hanging when the first auth state is an error

packages/firebase_auth/firebase_auth_web/lib/firebase_auth_web.dart

@@ -84,7 +84,10 @@ class FirebaseAuthWeb extends FirebaseAuthPlatform {
             }
           }
         }
-        await authDelegate.onWaitInitState();
+        await guardAuthExceptions(
+          authDelegate.onWaitInitState,
+          auth: authDelegate,
+        );
       },
     );
     FirebaseAuthPlatform.instance = FirebaseAuthWeb.instance;

packages/firebase_auth/firebase_auth_web/lib/src/interop/auth.dart

@@ -383,19 +383,29 @@ class Auth extends JsObjectWrapper<auth_interop.AuthJsImpl> {
   /// To preserve behavior on web and mobile we store the initial user
   /// in `_initUser` and add it manually to the `_changeController`.
   Future<void> onWaitInitState() async {
-    final completer = Completer();
+    final completer = Completer<void>();
     final nextWrapper = (auth_interop.UserJsImpl? user) {
       _initUser = User.getInstance(user);
-      completer.complete();
+      if (!completer.isCompleted) {
+        completer.complete();
+      }
     };
 
-    final errorWrapper = (JSAny e) => _changeController!.addError(e);
+    final errorWrapper = (JSAny e) {
+      if (!completer.isCompleted) {
+        completer.completeError(e);
+      }
+      _changeController?.addError(e);
+    };
 
     final unsubscribe =
         jsObject.onAuthStateChanged(nextWrapper.toJS, errorWrapper.toJS);
 
-    await completer.future;
-    unsubscribe.callAsFunction();
+    try {
+      await completer.future;
+    } finally {
+      unsubscribe.callAsFunction();
+    }
   }
 
   JSFunction? _onAuthUnsubscribe;

packages/firebase_auth/firebase_auth_web/test/on_wait_init_state_test.dart

@@ -0,0 +1,65 @@
+// Copyright 2026 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@TestOn('browser')
+library;
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import 'package:firebase_auth_web/src/interop/auth.dart' as auth_interop;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  /// Minimal stand-in for the JS `Auth` object: `onAuthStateChanged` invokes
+  /// either the next or the error callback once, and records unsubscribe.
+  ({auth_interop.Auth auth, bool Function() unsubscribed}) fakeAuth({
+    JSObject? error,
+  }) {
+    var unsubscribed = false;
+    final jsAuth = JSObject();
+    jsAuth['onAuthStateChanged'] = (JSFunction next, JSFunction? onError) {
+      if (error != null) {
+        onError!.callAsFunction(null, error);
+      } else {
+        // ignore: avoid_redundant_argument_values
+        next.callAsFunction(null, null);
+      }
+      return (() {
+        unsubscribed = true;
+      }).toJS;
+    }.toJS;
+    return (
+      auth: auth_interop.Auth.getInstance(jsAuth as auth_interop.AuthJsImpl),
+      unsubscribed: () => unsubscribed,
+    );
+  }
+
+  test('onWaitInitState completes when the first auth state arrives', () async {
+    final fake = fakeAuth();
+
+    await fake.auth.onWaitInitState();
+
+    expect(fake.unsubscribed(), isTrue);
+  });
+
+  test(
+    'onWaitInitState fails instead of hanging when the first auth state is an error',
+    () async {
+      final error = JSObject();
+      error['code'] = 'auth/internal-error'.toJS;
+      final fake = fakeAuth(error: error);
+
+      await expectLater(
+        fake.auth.onWaitInitState().timeout(const Duration(seconds: 2)),
+        throwsA(
+          predicate<Object>(
+            (e) => (e as JSObject)['code'].dartify() == 'auth/internal-error',
+          ),
+        ),
+      );
+      expect(fake.unsubscribed(), isTrue);
+    },
+  );
+}