[SalesForce] Apex REST Services Challenge – Unclear on return format

This is to reference with Salesforce Apex REST Services Trailhead

Create an Apex REST service that returns an account and it's contacts.
Scenario: Create an Apex REST class that is accessible at '/Accounts//contacts'. The service will return the account's ID and Name plus the ID and Name of all contacts associated with the account. Write unit tests that achieve 100% code coverage for the class and run your Apex tests.

Code:

 @RestResource(urlMapping='/Accounts/*/contacts')
    global with sharing class AccountManager {
        @HttpGet
        /*
        global static List<Contact> getAccount() {
            RestRequest req = RestContext.request;
            String accId = req.requestURI.substringBetween('Accounts/', '/contacts');
            Account acc = [SELECT Id, Name, (SELECT Id, Name FROM Contacts) FROM Account WHERE Id = :accId];
            system.debug(acc);
            system.debug(acc.Contacts);
            return acc.Contacts;
        }
        */
        global static Account getAccount() {
            RestRequest req = RestContext.request;
            String accId = req.requestURI.substringBetween('Accounts/', '/contacts');
            Account acc = [SELECT Id, Name, (SELECT Id, Name FROM Contacts) FROM Account WHERE Id = :accId];
            system.debug(acc);
            system.debug(acc.Contacts);
            return acc;
        }
    }

Test Class:

@IsTest
private class AccountManagerTest{

    @isTest static void testAccountManager() {
        Id recordId = createTestRecord();

        Contact Cont1 = new Contact();
        objCont.LastName = 'ABC';
        objCont.AccountId = recordId ;
        insert Cont1;

        Contact Cont2 = new Contact();
        objCont2.LastName = 'XYZ';
        objCont2.AccountId = recordId ;
        insert Cont2 ;

        RestRequest request = new RestRequest();
        request.requestUri =
            'https://na1.salesforce.com/services/apexrest/Accounts/'+recordId+'/contacts';
        request.httpMethod = 'GET';
        RestContext.request = request;
       Account lst = AccountManager.getAccount();
       /* Tried to return Contact List but failed Trailhead Verification
       List<Contact> lst = new List<Contact>();
       lst = AccountManager.getAccount();
       */
    }

    static Id createTestRecord() {
        Account accTest = new Account(Name='Test Record');
        insert accTest ;
        return accTest.Id;
    }          

}

I tried to verify with this apex class(code coverage 100%) but verification is failing. So I had tried to return "acc.Contacts"(which is commented in my class). Still verification is failing.

Error Message:

Please guide me where I am going wrong.