[ECS] [request]: Draining ECS container instance should drain target group before task is stopped

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**Tell us about your request**
When a ECS container instance is drained, the target group associated (if the task is in a service) should be drained before the task is stopped, in order to prevent service outages.

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I cannot solve this by my own because the task always stops before I can do anything.

The request concerns ECS container-instance draining and associated target groups, but the payload names no repository files, tests, or implementation entry points. Start by identifying where ECS draining behavior is defined; done would require ensuring the target group drains before the task stops without causing service outages.

Written by the indexing model from the issue text.