[ECS] [request]: Add a stopCode for ELB healthcheck terminated tasks

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**Tell us about your request**
It would be helpful for scripting if there was a stopCode on stopped tasks that were terminated because they failed ELB health checks. Currently the only indication is the stoppedReason message (which I assume is less set in stone).

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Rolling my own rollback mechanism for Fargate if health checks start failing on stack creation

**Are you currently working around this issue?**
Checking the stoppedReason

**Additional context**
Anything else we should know?

**Attachments**
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The request names ECS (Fargate) and ELB health checks but provides no repository files, tests, or entry points. Start by locating how stopped-task stop codes and ELB health-check termination are represented; done means these tasks expose an explicit stopCode that can support rollback scripting.

Written by the indexing model from the issue text.