Remove the transient keyword. It causes the variable not to get serialized.

Using the transient Keyword

Use the transient keyword to declare instance variables that can't be saved, and shouldn't be transmitted as part of the view state for a Visualforce page. For example:

Transient Integer currentTotal;

You can also use the transient keyword in Apex classes that are serializable, namely in controllers, controller extensions, or classes that implement the Batchable or Schedulable interface. In addition, you can use transient in classes that define the types of fields declared in the serializable classes.

Problem is...you can't serialize the request and response classes. You might find something like the below helpful:

Serializable Request

public class GenericRequest
{
    String requestType;
    String requestMethod;
    String requestURI;
    String requestParams;
    String requestHeaders;
    String requestBody;
    public GenericRequest(RestRequest request)
    {
        this.requestType = 'Inbound';
        this.requestMethod = request.httpMethod;
        this.requestURI = request.requestURI;
        this.requestParams = (request.params != null) ? JSON.serialize(request.params) : null;
        this.requestHeaders = (request.headers != null) ? JSON.serialize(request.headers) : null;
        this.requestBody = (request.requestBody != null) ? request.requestBody.toString() : null;
    }
    public GenericRequest(HttpRequest request)
    {
        this.requestMethod = request.getMethod();
        this.requestURI = request.getEndpoint();
        this.requestBody = request.getBody();
        this.requestType = 'Outbound';
    }
}

Serializable Response

public class GenericResponse
{
    String responseBody;
    Integer responseStatus;
    public GenericResponse(RestResponse response)
    {
        if (response == null) return;
        responseStatus = response.statusCode;
        if (response.responseBody != null)
            responseBody = response.responseBody.toString();
    }
    public GenericResponse(HttpResponse response)
    {
        if (response == null) return;
        responseStatus = response.getStatusCode();
        responseBody = response.getBody();
    }
}

Queueable Constructor

final GenericRequest request;
public MyQueueable(HttpRequest input)
{
    request = new GenericRequest(input);
}

@isTest classes are not supposed to be in the code coverage list, and should not count towards the code coverage rules (75% covered). Sometimes, however, things get messed up in the database, and you need to manually delete the erroneous coverage records.

To do this:

• Go to the Developer Console
• Go to the Query tab
• Enter the Query SELECT Id, NumLinesCovered FROM ApexCodeCoverageAggregate.
• Click on the "Use Tooling API" checkbox
• Select all rows
• Click Delete to delete the rows
• Click on Test | Clear Test Coverage.
• Run your tests again.

Again, your MockGenerator class should not have coverage, and should not count against the coverage requirements. If they are, you need to force the system to exclude them by deleting the relevant records.