[cdk diff] Modifications to stack-level tags do not surface in cdk diff results, leading to unexpected updates

### Describe the bug

When you make a change to a stack-level tag in CDK, `cdk diff` returns `There were no differences` even though changes to stack-level tags do in fact results in updates to resources. This results in inaccurate diff results that can lead to unexpected updates.

In comparison - When you make a change to a stack-level tag and then create a CloudFormation ChangeSet, CloudFormation **does** report this as a change. It lists a `Modify` operation for all supported resources.

The `cdk diff` command creates (and then immediately deletes) a CloudFormation ChangeSet, and this ChangeSet does accurately report the `Modify` behavior, but the actual `cdk diff` results in the CDK CLI still returns `no differences` despite this. I have provided screenshots of this below.

**CloudFormation ChangeSet:**

**CDK Diff Results**:

In other words, this doesn't appear to be an issue or limitation with the CloudFormation ChangeSet behavior. This specifically looks like a limitation with how CDK diff parses and returns ChangeSet results when stack-level tags are involved.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

CDK diff should accurately report changes to resources when changes are made to stack-level tags.

### Current Behavior

CDK diff incorrectly returns "There were no differences" despite the CloudFormation ChangeSet reporting differences.

### Reproduction Steps

1. Create a CDK stack. As an example, I made one that just creates an SNS topic:

```
import * as cdk from 'aws-cdk-lib/core';
import * as sqs from 'aws-cdk-lib/aws-sqs';
import { Construct } from 'constructs';

export class CdkDiffReplicationStack extends cdk.Stack {
constructor(scope: Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);

new sqs.Queue(this, 'Queue');
}
}
```

2. Define one or more stack tags. For example:

```
#!/usr/bin/env node
import * as cdk from 'aws-cdk-lib/core';
import { CdkDiffReplicationStack } from '../lib/cdk_diff_replication-stack';

const app = new cdk.App();

new CdkDiffReplicationStack(app, 'CdkDiffReplicationStack', {
// Specialize for the account/region from the current CLI configuration.
env: {
account: process.env.CDK_DEFAULT_ACCOUNT,
region: process.env.CDK_DEFAULT_REGION,
},

// stack-level tag
tags: {
DiffReplicationTag: 'Original',
},
});
```

3. Deploy the stack
4. Make a change to the stack-level tag in the CDK code:

```
tags: {
DiffReplicationTag: 'Changed', //<---Changed the value
},
```

5. Run CDK diff - Observe that the command returns `There were no differences`.
6. In the CloudFormation console, manually create a CloudFormation ChangeSet. Use the same template but change the value of the stack-level tag. Observe that CloudFormation does report changes for these resources.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.1141.0

### Framework Version

_No response_

### Node.js Version

v24.14.0

### OS

Windows 10

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start by reproducing the issue with the example in lib/cdk_diff_replication-stack and the stack-level tags shown in the app entry point, then run cdk diff and compare its output with a CloudFormation ChangeSet. Trace how the CLI parses ChangeSet results; done means cdk diff reports the resource modifications caused by changing the tag value.

Written by the indexing model from the issue text.