Address undercoverage for diagram proxy
What does this MR do and why?
Resolves Address undercoverage for !223314 (#593052 - closed) by adding coverage for the missed paths in Add diagram proxy to route Kroki/PlantUML reque... (!223314 - merged) noted in https://gitlab.com/gitlab-org/gitlab/-/jobs/13442626929.
How to set up and validate locally
This is just adding specs, there's no change in behaviour whatsoever. Specs are run by CI so I don't think there's any need for local setup/validation.
MR acceptance checklist
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Edited by Asherah Connor