core: CloudFormation-Validate::W2001 false positive

### Describe the bug

SSM parameter used for ECS task secret generates false positive W2001 warning.

eg:
```
task.add_container(
...
secrets={
"MONGO_URI": ecs.Secret.from_ssm_parameter(
ssm.StringParameter.from_string_parameter_name(
self,
"mongodb-uri",
string_parameter_name=f"/path-to/mongodb-uri",
)
),
)
```

Would generate a warning:
WARNING Parameter 'mongodburiParameter' is not referenced anywhere in the template (CloudFormation Validate)
...
Acknowledge with 'CloudFormation-Validate::W2001'

### Regression Issue

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

### Last Known Working CDK Library Version

v2.261.0

### Expected Behavior

No warning.

### Current Behavior

WARNING Parameter 'mongodburiParameter' is not referenced anywhere in the template (CloudFormation Validate)
...
Acknowledge with 'CloudFormation-Validate::W2001'

### Reproduction Steps

See above.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

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

v2.262.0

### AWS CDK CLI version

2.1133.0 (build 712d792)

### Node.js Version

v24.18.0

### OS

Linux

### Language

Python

### Language Version

Python 3.12.3

### Other information

_No response_

Start by reproducing the reported ECS task secret example with aws-cdk-lib v2.262.0 and inspect where the CloudFormation-Validate::W2001 warning is produced. Compare the result with v2.261.0 and add a regression test for the SSM-backed ECS secret; done means the generated template no longer reports the parameter as unreferenced.

Written by the indexing model from the issue text.