Fix is_async status of agent execution task

[rithin-pullela-aws]

Description

When an agent is executed in async mode and the task is retrieved, we see the is_async parameter in response as false.

GET /_plugins/_ml/tasks/<task_id>
{
    ...
    "is_async": false,
    "response": {
        ...
    }
}

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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