lambda-durable-scheduled-tasks-sam: Fix empty taskId in async API Gateway response

[kakakakakku]

Issue #, if available:
N/A

Description of changes:
Hi😀 Thanks for the useful patterns!

While testing lambda-durable-scheduled-tasks-sam, the API response returns blank taskId. Because Lambda functions with asynchronous invoke does not return response directly.
https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-integration-async.html

$ curl -X POST ${API_ENDPOINT}/tasks \
  -H "Content-Type: application/json" \
  -d '{
    "config": {
      "reportType": "weekly",
      "dataSource": "analytics",
      "notifyEmail": "admin@example.com"
    }
  }'
{
  "taskId": "",
  "status": "INITIALIZED",
  "message": "Task started successfully"
}

But README is mentioned with taskId response.

serverless-patterns/lambda-durable-scheduled-tasks-sam/README.md

Lines 90 to 97 in a56877a

	Response:
	```json
	{
	"taskId": "TASK-1733328000000",
	"status": "INITIALIZED",
	"message": "Task started successfully"
	}
	```

Check

I fixed mapping template and it works good.

$ curl -X POST ${API_ENDPOINT}/tasks \
  -H "Content-Type: application/json" \
  -d '{
    "config": {
      "reportType": "weekly",
      "dataSource": "analytics",
      "notifyEmail": "admin@example.com"
    }
  }'
{
  "taskId": "TASK-1769224879385",
  "status": "INITIALIZED",
  "message": "Task started successfully"
}

Thank you😀

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