Add a QA test for malformed URLs

What does this MR do and why?

Because of https://github.com/puma/puma/pull/3094, Puma v6.4.0 returns the status code sent from the low-level handler instead of Puma's recommended status code. This caused malformed URLs to return a 500 error when previously it would return a 400 Bad Request error, which skewed error metrics.

To catch this issues in the future, add a gitlab-qa test that attempts to issue a malformed URL.

Relates to gitlab-com/gl-infra/production#16417 (closed)

How to set up and validate locally

In the qa directory, run:

QA_GITLAB_URL=https://gitlab.com bundle exec rspec qa/specs/features/api/3_create/merge_request/view_merge_requests_spec.rb

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.