(cdk): can't use hotswap when Custom::AWS is present in template

### Describe the bug

`Could not perform a hotswap deployment, because the CloudFormation template could not be resolved: We don't support attributes of the 'Custom::AWS' resource. This is a CDK limitation. Please report it at https://github.com/aws/aws-cdk/issues/new/choose`.

The template i have does contain a Custom::AWS resource but cdk diff does not detect any changes to it.

```
[~] AWS::CloudFormation::Stack a.NestedStack/a.NestedStackResource aNaNREEA7D805
└─ [~] TemplateURL
└─ [~] .Fn::Join:
└─ @@ -5,6 +5,6 @@
[ ] {
[ ] "Ref": "AWS::URLSuffix"
[ ] },
[-] "/cdk-hnb659fds-assets--/.json"
[+] "/cdk-hnb659fds-assets--/.json"
[ ] ]
[ ] ]

Stack stack-bNbNRE1B71636-1XZCJ7DPIISJG
There were no differences
Stack stack-aNaNREEA7D805-VZ5MPHGEVZF3
Resources
[~] AWS::Lambda::Function a/fn fn588520B7
├─ [~] Code
│ └─ [~] .S3Key:
│ ├─ [-].zip
│ └─ [+] .zip
└─ [~] Metadata
└─ [~] .aws:asset:path:
├─ [-] asset.
└─ [+] asset.
```

### Regression Issue

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

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

If possible it should be possible for this check to be bypassed.

### Current Behavior

`Could not perform a hotswap deployment, because the CloudFormation template could not be resolved: We don't support attributes of the 'Custom::AWS' resource. This is a CDK limitation. Please report it at https://github.com/aws/aws-cdk/issues/new/choose`.

### Reproduction Steps

Create a simple stack with an AwsCustomResource and a lambda function. Update code in lambda function and attempt to deploy with --hotswap.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

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

2.218.0

### AWS CDK CLI version

2.1031.0 (build 3d7b09b)

### Node.js Version

v22.18.0

### OS

MacOS 26

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start with the --hotswap deployment path that resolves the CloudFormation template, using the reported reproduction: a stack with AwsCustomResource and a Lambda function whose code changes. Verify the behavior when Custom::AWS is present but unchanged, and consider the issue's expected outcome: allowing the hotswap check to be bypassed or proceed safely.

Written by the indexing model from the issue text.