[ECR] [request]: Give a semantically accurate HTTP response when a docker push would exceed a repository's # of images quota

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**Tell us about your request**
If a docker push would take an ECR repository over the quota for that repository's max number of images, AWS responds with a "403 Unauthorized". This makes the error difficult to debug because it implies the user's AWS credentials are invalid or don't have the correct IAM role. AWS should return either a 400 or a 406.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I would just like AWS to actually describe the problem with the authorized but invalid API operation I'm trying to perform instead of relying on me to figure out what's going wrong. I can't imagine how hard this error would be to figure out if our image quota wasn't the default (10_000).

The payload names no repository files, tests, or entry points; start by checking how this roadmap repository records ECR requests and whether an implementation repository is identified. Done would mean AWS ECR returns a semantically accurate 400 or 406 instead of 403 when a push exceeds the repository image quota.

Written by the indexing model from the issue text.