Fix VSA consistency worker cursor generator
What does this MR do and why?
This MR fixes a typo in the VSA ConsistencyWorker and adds an extra test case. Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3329501/?query=is%3Aunresolved%20ConsistencyWorker
The ConsistencyWorker got a wrong cursor hash with the wrong column (issue_id instead of merge_request_id) and causes a runtime error.
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