"Render state machine graph from CDK application" always fails

## Problem

I have defined two Step Functions state machines in my CDK (Python) application.

When I try to use the render option the preview panel opens but with this error:
```
Errors detected. Cannot preview.
```

## Steps to reproduce the issue

Open my project then either use the AWS browser and click preview icon:

or Cmd+Shift+P and select "AWS: Render state machine graph from CDK application"

## Expected behavior

See a graph

## System details (run the `AWS: About Toolkit` command)

- OS: macOS 14.3.1
- Visual Studio Code version: 1.87.2 (Universal)
- AWS Toolkit version: v2.15.0

Reproduce the failure in the AWS Toolkit for VS Code by opening the project and running “AWS: Render state machine graph from CDK application,” using the two-state-machine Python CDK setup described. Compare the preview panel error with the expected graph output and use the AWS Toolkit version and system details to narrow the behavior. Done means the preview renders the state machine graph instead of showing “Errors detected. Cannot preview.”

Written by the indexing model from the issue text.