fix: apply tags to nested stack (#19128)

When a nested stack gets a custom tags the tag gets applied only to the resources defined in the stack and not the stack itself. This change fixes this issue and applies the tag to the nested stack as well. This was caused by NestedStack subclassing Stack but not using the NestedStack.tags in the `CfnStack` (as it  accepts only `CfnTag[]` and not `TagManager`)

This change updates the `_prepareTemplateAsset` to get all the tags added to the `CfnStack`  from `NestedStack.tag`  before rendering the stack
    
fixes #17463


----

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

Author: yuth

packages/@aws-cdk/aws-cloudformation/test/nested-stack.test.ts

@@ -4,7 +4,7 @@ import { Template } from '@aws-cdk/assertions';
 import * as s3_assets from '@aws-cdk/aws-s3-assets';
 import * as sns from '@aws-cdk/aws-sns';
 import { describeDeprecated } from '@aws-cdk/cdk-build-tools';
-import { App, CfnParameter, CfnResource, ContextProvider, LegacyStackSynthesizer, Names, Stack } from '@aws-cdk/core';
+import { App, CfnParameter, CfnResource, ContextProvider, LegacyStackSynthesizer, Names, Stack, Tags } from '@aws-cdk/core';
 import { NestedStack } from '../lib/nested-stack';
 
 // keep this import separate from other imports to reduce chance for merge conflicts with v2-main
@@ -1085,4 +1085,49 @@ describeDeprecated('NestedStack', () => {
     });
   });
 
+  test('nested stack should get the tags added in root stack', () =>{
+    const app = new App();
+    const parentStack = new Stack(app, 'parent-stack');
+    const nestedStack = new NestedStack(parentStack, 'MyNestedStack');
+
+    // add tags
+    Tags.of(nestedStack).add('tag-1', '22');
+    Tags.of(nestedStack).add('tag-2', '33');
+
+    new sns.Topic(nestedStack, 'MyTopic');
+
+    // THEN
+    Template.fromStack(parentStack).hasResourceProperties(
+      'AWS::CloudFormation::Stack',
+      {
+        Tags: [
+          {
+            Key: 'tag-1',
+            Value: '22',
+          },
+          {
+            Key: 'tag-2',
+            Value: '33',
+          },
+        ],
+      },
+    );
+
+    Template.fromStack(nestedStack).hasResourceProperties(
+      'AWS::SNS::Topic',
+      {
+        Tags: [
+          {
+            Key: 'tag-1',
+            Value: '22',
+          },
+          {
+            Key: 'tag-2',
+            Value: '33',
+          },
+        ],
+      },
+    );
+  });
+
 });

packages/@aws-cdk/core/lib/nested-stack.ts

@@ -205,6 +205,16 @@ export class NestedStack extends Stack {
       return false;
     }
 
+    // When adding tags to nested stack, the tags need to be added to all the resources in
+    // in nested stack, which is handled by the `tags` property, But to tag the
+    //  tags have to be added in the parent stack CfnStack resource. The CfnStack resource created
+    // by this class dont share the same TagManager as that of the one exposed by the `tag` property of the
+    //  class, all the tags need to be copied to the CfnStack resource before synthesizing the resource.
+    // See https://github.com/aws/aws-cdk/pull/19128
+    Object.entries(this.tags.tagValues()).forEach(([key, value]) => {
+      this.resource.tags.setTag(key, value);
+    });
+
     const cfn = JSON.stringify(this._toCloudFormation());
     const templateHash = crypto.createHash('sha256').update(cfn).digest('hex');