Fix flaky issue participants specs
What does this MR do and why?
Make sure other issues with a matching IID don't exist in the DB on specs that expect a 404 is using the issue's ID instead of IID. The problem was that sometimes the ID would match with another issue's IID and return a 200.
MR acceptance checklist
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How to set up and validate locally
Failure is reproducible by running
bundle exec rspec spec/requests/api/issues/get_project_issues_spec.rb --seed 3228
Related to #444827 (closed)