feat: add TOTP MFA support and improve Auth test coverage

packages/dart_firebase_admin/example/lib/main.dart

@@ -28,7 +28,7 @@ Future<void> authExample(FirebaseApp admin) async {
     if (e.errorCode == AuthClientErrorCode.userNotFound) {
       print('> User not found, creating new user\n');
       user = await auth.createUser(
-        CreateRequest(email: '[email protected]', password: 'Test@123'),
+        CreateRequest(email: '[email protected]', password: 'Test@12345'),
       );
     } else {
       print('> Auth error: ${e.errorCode} - ${e.message}');

packages/dart_firebase_admin/lib/src/app_check/app_check.dart

@@ -29,7 +29,24 @@ class AppCheck implements FirebaseService {
       _appCheckTokenVerifier = AppCheckTokenVerifier(app);
 
   @internal
-  AppCheck.internal(
+  factory AppCheck.internal(
+    FirebaseApp app, {
+    AppCheckRequestHandler? requestHandler,
+    AppCheckTokenGenerator? tokenGenerator,
+    AppCheckTokenVerifier? tokenVerifier,
+  }) {
+    return app.getOrInitService(
+      FirebaseServiceType.appCheck.name,
+      (app) => AppCheck._internal(
+        app,
+        requestHandler: requestHandler,
+        tokenGenerator: tokenGenerator,
+        tokenVerifier: tokenVerifier,
+      ),
+    );
+  }
+
+  AppCheck._internal(
     this.app, {
     AppCheckRequestHandler? requestHandler,
     AppCheckTokenGenerator? tokenGenerator,

packages/dart_firebase_admin/lib/src/auth/auth.dart

@@ -4,21 +4,40 @@ part of '../auth.dart';
 /// An Auth instance can have multiple tenants.
 class Auth extends _BaseAuth implements FirebaseService {
   /// Creates or returns the cached Auth instance for the given app.
-  factory Auth(
+  factory Auth(FirebaseApp app) {
+    return app.getOrInitService(FirebaseServiceType.auth.name, Auth._);
+  }
+
+  Auth._(FirebaseApp app)
+    : super(app: app, authRequestHandler: AuthRequestHandler(app));
+
+  @internal
+  factory Auth.internal(
     FirebaseApp app, {
-    @internal AuthRequestHandler? requestHandler,
+    AuthRequestHandler? requestHandler,
+    FirebaseTokenVerifier? idTokenVerifier,
+    FirebaseTokenVerifier? sessionCookieVerifier,
   }) {
     return app.getOrInitService(
       FirebaseServiceType.auth.name,
-      (app) => Auth._(app, requestHandler: requestHandler),
+      (app) => Auth._internal(
+        app,
+        requestHandler: requestHandler,
+        idTokenVerifier: idTokenVerifier,
+        sessionCookieVerifier: sessionCookieVerifier,
+      ),
     );
   }
 
-  Auth._(FirebaseApp app, {@internal AuthRequestHandler? requestHandler})
-    : super(
-        app: app,
-        authRequestHandler: requestHandler ?? AuthRequestHandler(app),
-      );
+  Auth._internal(
+    FirebaseApp app, {
+    AuthRequestHandler? requestHandler,
+    super.idTokenVerifier,
+    super.sessionCookieVerifier,
+  }) : super(
+         app: app,
+         authRequestHandler: requestHandler ?? AuthRequestHandler(app),
+       );
 
   @override
   Future<void> delete() async {