Fix flaky test markdown-links-414-docs

The merge lane saw `markdown-links-414-docs` fail and then pass on a rerun of the same tree. - Test: `markdown-links-414-docs` - Tree: `e06da000481b70151344a28664a2906bbbdecb6c` - Lane task: 13844 - Flake record: 01a1a685-5e9e-73f8-b794-2d72136ec696 Make it deterministic; do not delete it unless it tests nothing.

The Markdown links stage no longer fails when a proxy reports that the origin gave no answer: 502/503/504 and Cloudflare 520–524/530 now count as unreachable, like Status 0, while 4xx and 500 still fail. The flake was a 522 from unicode.org. Quick gate green; PR #7961 queued to the merge lane. Memory: none applied.