core: CloudFormation-Validate::W9007 false positive when subnet id is Fn::GetStackOutput

### Describe the bug

`CloudFormation-Validate::W9007` is reported on Application Load Balancer, ElastiCache SubnetGroup, and RDS SubnetGroup when subnets are imported from another stack using `Fn::GetStackOutput`.

Feature flag `@aws-cdk/core:defaultCrossStackReferences` is `weak`.

```ts
new elbv2.ApplicationLoadBalancer(stack, 'ALB', {
vpc: anotherStack.vpc,
...
});
```

Template:
```json5
{
"Resource": {
"ALBXXXX": {
"Type": "ElasticLoadBalancingV2::LoadBalancer",
"Properties": {
// ...
"Subnets": [
{
"Fn::GetStackOutput": {
"StackName": "XXX",
"Region": "ap-northeast-1",
"OutputName": "PublishOutputRefVpcIsolatedSubnet1SubnetXXX"
}
},
{
"Fn::GetStackOutput": {
"StackName": "XXX",
"Region": "ap-northeast-1",
"OutputName": "PublishOutputRefVpcIsolatedSubnet2SubnetXXX"
}
},
{
"Fn::GetStackOutput": {
"StackName": "XXX",
"Region": "ap-northeast-1",
"OutputName": "PublishOutputRefVpcIsolatedSubnet3SubnetXXX"
}
}
],
"Type": "application"
}
}
}
}
```

> WARNING Subnets: Array property 'Subnets' contains duplicate values (CloudFormation Validate)
> aws-cdk-lib.aws_elasticloadbalancingv2.CfnLoadBalancer
> Acknowledge with 'CloudFormation-Validate::W9007'

Same issue on aws-cdk-lib.aws_elasticache.CfnSubnetGroup and aws-cdk-lib.aws_rds.CfnDBSubnetGroup.

### Regression Issue

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

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

No warnigns are reported.

### Current Behavior

See description above.

### Reproduction Steps

See description above.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

2.262.1

### AWS CDK CLI version

2.1133.0 (build 712d792)

### Node.js Version

v24.18.0

### OS

Linux

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Reproduce the warning with the supplied CloudFormation template and the Application Load Balancer, ElastiCache SubnetGroup, or RDS SubnetGroup examples using Fn::GetStackOutput. Trace the CloudFormation-Validate::W9007 handling and verify that subnet values imported this way no longer produce duplicate-value warnings.

Written by the indexing model from the issue text.