feat: add error code for error while throttling

Description

Use HTTP code 422 (UnprocessableContentMessageResponse) and add special error code to represent request that are timeout due to throttling

[x] I have clicked on "allow edits by maintainers".
[ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
[x] The correct base branch is being used, if not main
[ ] I have added tests to validate that the change in functionality is working as expected