chore: Mark mocked HTTP client final to silence Sendable warning (#1023)

Author: jbelkins

Tests/ClientRuntimeTests/NetworkingTests/Http/MockHttpClientEngine.swift

@@ -1,15 +1,17 @@
-/*
- * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
- * SPDX-License-Identifier: Apache-2.0.
- */
+//
+// Copyright Amazon.com Inc. or its affiliates.
+// All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
 
 import Smithy
 import SmithyHTTPAPI
 import Foundation
 import AwsCommonRuntimeKit
 @testable import ClientRuntime
 
-class MockHttpClientEngine: HTTPClient {
+final class MockHttpClientEngine: HTTPClient {
     func successHttpResponse(request: SmithyHTTPAPI.HTTPRequest) -> HTTPResponse {
         return HTTPResponse(headers: request.headers, body: ByteStream.empty, statusCode: HTTPStatusCode.ok)
     }