fix(integ-tests): http flattenResponse (#30361)

### Issue # (if applicable)

Closes #30327

### Reason for this change

There was a difference in the behavior of SDK and HTTP integration attribute extraction with the `getAtt` and `getAttString` methods. `awsApiCall` properly implemented and returned JSONPath-ish values by using a `flattenResponse` property. This PR adds the same functionality to `httpApiCall`

### Description of changes

Added an implemented `flattenResponse` in the `HttpHandler` custom resource

### Description of how you validated changes

Updated integ and unit tests

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/integ-tests-alpha/lib/assertions/http-call.ts

@@ -32,6 +32,7 @@ export class HttpApiCall extends ApiCallBase {
         parameters: props,
         expected: Lazy.any({ produce: () => this.expectedResult }),
         stateMachineArn: Lazy.string({ produce: () => this.stateMachineArn }),
+        flattenResponse: Lazy.string({ produce: () => this.flattenResponse }),
         salt: Date.now().toString(),
       },
       resourceType: `${HTTP_RESOURCE_TYPE}${name}`.substring(0, 60),

packages/@aws-cdk/integ-tests-alpha/lib/assertions/providers/lambda-handler/http.ts

@@ -2,11 +2,13 @@
 // eslint-disable-next-line import/no-extraneous-dependencies
 import fetch, { Response } from 'node-fetch';
 // TODO: can use builtin fetch on node18
+import { flatten } from '@aws-cdk/aws-custom-resource-sdk-adapter';
 import { CustomResourceHandler } from './base';
 import { HttpRequest, HttpResponseWrapper } from './types';
+import { deepParseJson } from './utils';
 
-export class HttpHandler extends CustomResourceHandler<HttpRequest, HttpResponseWrapper> {
-  protected async processEvent(request: HttpRequest): Promise<HttpResponseWrapper> {
+export class HttpHandler extends CustomResourceHandler<HttpRequest, HttpResponseWrapper | { [key: string]: unknown }> {
+  protected async processEvent(request: HttpRequest): Promise<HttpResponseWrapper | { [key: string]: unknown }> {
     console.log('request', request);
     const response: Response = await fetch(request.parameters.url, request.parameters.fetchOptions);
     const result: any = {
@@ -22,8 +24,17 @@ export class HttpHandler extends CustomResourceHandler<HttpRequest, HttpResponse
     } catch (e) {
       // Okay
     }
-    return {
-      apiCallResponse: result,
-    };
+
+    let resp: HttpResponseWrapper | { [key: string]: unknown };
+    if (request.flattenResponse === 'true') {
+      // Flatten and explode JSON fields
+      resp = flatten(deepParseJson({ apiCallResponse: result }));
+    } else {
+      // Otherwise just return the response as-is, without exploding JSON fields
+      resp = { apiCallResponse: result };
+    }
+    console.log(`Returning result ${JSON.stringify(resp)}`);
+
+    return resp;
   }
 }

packages/@aws-cdk/integ-tests-alpha/lib/assertions/providers/lambda-handler/types.ts

@@ -25,6 +25,22 @@ export interface HttpRequest {
    * Parameters from the custom resource
    */
   readonly parameters: HttpRequestParameters;
+
+  /**
+   * Whether or not to flatten the response from the HTTP request
+   *
+   * Valid values are 'true' or 'false' as strings
+   *
+   * Typically when using an HttpRequest you will be passing it as the
+   * `actual` value to an assertion provider so this would be set
+   * to 'false' (you want the actual response).
+   *
+   * If you are using the HttpRequest to perform more of a query to return
+   * a single value to use, then this should be set to 'true'.
+   *
+   * @default 'false'
+   */
+  readonly flattenResponse?: string;
 }
 
 /**