versionDescription not returned for stepfunctions.describe_state_machine using version ARN

[ekolve]

When running the following sequence under moto-5.0.26.dev0:

import boto3
client = boto3.client("stepfunctions", region_name='us-west-2)
client.create_state_machine(
            name='somestatemachine',
            definition=json.dumps({}),
            roleArn=<role arn>,
            type="STANDARD",
            versionDescription="first version of statemachine",
            publish=True,
        )
version_arn = res["stateMachineVersionArn"]

followed by this call:

res = client.describe_state_machine(stateMachineArn=version_arn)
print(res["versionDescription"]) # KeyError

The versionDescription is not returned, but this should be returned when calling describe_state_machine with an ARN that includes a version.