Mocking DynamoDB in an AWS Lambda Unit Test

I have an AWS Serverless API that performs CRUD operations on a DynamoDB table. The functions work as expected, but I am having trouble trying to add unit tests to support our CI/CD pipeline. The tests fail because they can't create the DynamoDB context and I don't want them to as they would be integration tests. How can I refactor the function so that I can mock IDynamoDBContext and/or write a test for this function?

Note: I am using Nunit and NSubstitute for testing. I'm open to other testing libraries if it makes this easier. I have seen examples of mocking IDynamoDBContext and subclassing AsyncSearch, but I can't inject IDynamoDBContext so that will not work with the code as written.

A simplified version of my Read Lambda function currently looks like this:

using System.Text.Json;using Amazon.DynamoDBv2;using Amazon.DynamoDBv2.DataModel;using Amazon.Lambda.APIGatewayEvents;using Amazon.Lambda.Core;using Tenants.Domain.Entities;// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]namespace Tenants.Read;public class ReadFunction{    public async Task<APIGatewayProxyResponse> FunctionHandler(APIGatewayProxyRequest apigProxyEvent, ILambdaContext context)    {        string response;        int statusCode = 200;        try        {            DynamoDBContext dbContext = CreateDynamoDbContext();            List<Tenant> tenants = await dbContext.ScanAsync<Tenant>([]).GetRemainingAsync();            response = JsonSerializer.Serialize(tenants);        }        catch (Exception ex)        {            statusCode = 500;            response = JsonSerializer.Serialize(ex.Message);        }        return new APIGatewayProxyResponse        {            Body = response,            StatusCode = statusCode,            Headers = new Dictionary<string, string> { { "Content-Type", "application/json" } }        };    }    private static DynamoDBContext CreateDynamoDbContext()    {        AmazonDynamoDBConfig config = new() { AuthenticationRegion = "us-east-1" };        AmazonDynamoDBClient client = new(config);        return new DynamoDBContext(client);    }}

The beginning of my unit test looks like this:

using System.Text.Json;using Amazon.Lambda.APIGatewayEvents;using Amazon.Lambda.TestUtilities;using NUnit.Framework;namespace Tenants.Read.Tests;[TestFixture]public class ReadFunctionTest{    [Test]    public async Task FunctionHandler_Returns200()    {        // Arrange        var function = new ReadFunction();        var context = new TestLambdaContext();        var request = new APIGatewayProxyRequest();        // Act        APIGatewayProxyResponse response = await function.FunctionHandler(request, context);        // Assert        Assert.That(response.StatusCode, Is.EqualTo(200));    }}