fix(crossRegionReference): error message missing stack information (#29961)

### Issue #29699

Closes #29699

### Reason for this change

The logic was incorrectly using `Object.entries()` on a Map. 

### Description of changes

This is a straighforward change to properly handle the map, and an update to tests.

### Description of how you validated changes

Updated test

### 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*

Author: andreialecu

packages/@aws-cdk/custom-resource-handlers/lib/core/cross-region-ssm-writer-handler/index.ts

@@ -102,8 +102,8 @@ async function throwIfAnyInUse(ssm: SSM, parameters: CrossRegionExports): Promis
   }));
 
   if (tagResults.size > 0) {
-    const message: string = Object.entries(tagResults)
-      .map((result: [string, string[]]) => `${result[0]} is in use by stack(s) ${result[1].join(' ')}`)
+    const message: string = Array.from(tagResults.entries())
+      .map((result) => `${result[0]} is in use by stack(s) ${Array.from(result[1]).join(' ')}`)
       .join('\n');
     throw new Error(`Exports cannot be updated: \n${message}`);
   }

packages/@aws-cdk/custom-resource-handlers/test/core/cross-region-ssm-writer-handler.test.ts

@@ -1,7 +1,7 @@
 /* eslint-disable import/no-extraneous-dependencies */
 import { InvalidResourceId } from '@aws-sdk/client-ssm';
-import { SSM_EXPORT_PATH_PREFIX } from '../../lib/core/types';
 import { handler } from '../../lib/core/cross-region-ssm-writer-handler/index';
+import { SSM_EXPORT_PATH_PREFIX } from '../../lib/core/types';
 
 let mockPutParameter: jest.Mock = jest.fn() ;
 let mocklistTagsForResource: jest.Mock = jest.fn();
@@ -96,7 +96,7 @@ describe('cross-region-ssm-writer entrypoint', () => {
       });
 
       // THEN
-      await expect(handler(event)).rejects.toThrow(/Exports cannot be updated/);
+      await expect(handler(event)).rejects.toThrow('Exports cannot be updated: \n/cdk/exports/MyStack/MyExport is in use by stack(s) MyStack');
     });
 
     test('Create event does not throw for new parameters', async () => {