Variable placeholder not recognised in state-machine.asl.json

## Problem

Where a state machine file with **.asl.json** exists, red lines appear in VS Code, stating the state machine json is not valid with a valid variable placeholder, and a visual representation of the state machine subsequently not being rendered.

```
"Wait": {
"Type": "Wait",
"Seconds": ${someVariablePlaceholder},
"Next": "Some step",
"QueryLanguage": "JSONata",
"Output": "{% $states.input %}"
},
```

## Steps to reproduce the issue

See above

## Expected behavior

## System details (run `AWS: About` and/or `Amazon Q: About`)

- OS: Mac OS X
- Visual Studio Code version: 1.120.0
- AWS Toolkit version: 4.5.0

Reproduce the problem in VS Code 1.120.0 with the provided state-machine.asl.json snippet and AWS Toolkit 4.5.0. Trace the validation and visual-rendering entry points for state machine files, then verify that the variable placeholder produces no invalid-JSON diagnostic and that the visual representation renders.

Written by the indexing model from the issue text.