(aws-stepfunctions): CustomState generates wrong code when using `Type: Choice`

### Describe the bug

I am using the `CustomState` construct to generate a `Choice` with `JSONata`. The problem is that since `CustomState` implements `INextable`, this adds an `End` property which is invalid for the `Choice` state.

### Regression Issue

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

### Last Known Working CDK Version

_No response_

### Expected Behavior

For this CDK construct

```
const shouldContinue = new sfn.CustomState(this, 'ShouldContinue', {
stateJson: {
Type: 'Choice',
QueryLanguage: 'JSONata',
Default: 'AnalysisComplete',
Choices: [
{
Next: 'ProcessDiscoveredUrls',
Condition:
'{% $count($states.input.nextBatch) > 0 and $count($states.input.visitedUrls) <= $states.input.maxPages %}',
},
],
},
});
```

I would expect, the proper JSON to be constructed without the `End` property.

### Current Behavior

This is the generated JSON

```
{
"End": true,
"Type": "Choice",
"QueryLanguage": "JSONata",
"Default": "AnalysisComplete",
"Choices":[{
"Next": "ProcessDiscoveredUrls",
"Condition": "{% $count($states.input.nextBatch) > 0 and $count($states.input.visitedUrls) <= $states.input.maxPages %}"
}]
}
```

### Reproduction Steps

This is easily reproduced using the construct above.

### Possible Solution

One solution would be adding an if statement to conditionally call `renderNextEnd()` in `toStateJson()` but then `CustomState` wouldn't really implement `INextable`. There needs to be a construct one level higher that doesn't implement `INextable`.

### Additional Information/Context

_No response_

### CDK CLI Version

2.173.2 (build f8e6207)

### Framework Version

_No response_

### Node.js Version

20

### OS

AL2

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at the CustomState implementation, especially toStateJson() and renderNextEnd(), which the issue identifies as the relevant entry points. Reproduce the supplied Choice/JSONata example and inspect the generated state JSON; done means the Choice output no longer contains the invalid End property while valid nextable states retain their expected behavior.

Written by the indexing model from the issue text.