Add 422 Unprocessable Submission response to /evaluate

[m-messer]

Summary

Adds a 422 Unprocessable Submission response to POST /evaluate. An evaluation service returns it when the request is valid but it can't process the submission content, for example an unparseable expression. The body is the standard ErrorResponse with code VALIDATION_ERROR.

At the moment a submission the service can't interpret has to come back as 500 INTERNAL_ERROR, which clients can't tell apart from a real service failure. A separate status lets clients show the learner a fix-your-input message.

Changes:

• New paths/evaluate/responses/422-UnprocessableSubmission.yml, in the same layout as the other evaluate responses.
• paths/evaluate/evaluate.yml lists "422" as a response for /evaluate.

Related issue

Mirrors the 422 handling added to the Lambda Feedback evaluation function layer in lambda-feedback/BaseEvalutionFunctionLayer#21.

Scope

• This pull request is focused on a single concern.
• The change was started from the latest main branch, or from a fork if direct branch creation is not available.

Validation

• npm run lint: valid. Its 11 warnings are all already on main; none come from this change.
• npm run bundle

Notes for reviewers

This only adds a response, so it's backwards compatible: existing services and clients keep working. It reuses the existing VALIDATION_ERROR code rather than adding a new one.

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