Fix syntax errors

Author: mansisampat

packages/auth/src/core/auth/auth_impl.ts

@@ -676,7 +676,7 @@ export class AuthImpl implements AuthInternal, _FirebaseService {
     nextOrObserver: NextOrObserver<FirebaseToken>,
     error?: ErrorFn,
     completed?: CompleteFn
-  ): Unsubscribe | undefined {
+  ): Unsubscribe {
     return this.registerStateListener(
       this.firebaseTokenSubscription,
       nextOrObserver,

packages/auth/src/model/auth.ts

@@ -85,7 +85,7 @@ export interface AuthInternal extends Auth {
     nextOrObserver: NextOrObserver<FirebaseToken>,
     error?: ErrorFn,
     completed?: CompleteFn
-  ): Unsubscribe | undefined;
+  ): Unsubscribe;
   _agentRecaptchaConfig: RecaptchaConfig | null;
   _tenantRecaptchaConfigs: Record<string, RecaptchaConfig>;
   _projectPasswordPolicy: PasswordPolicy | null;