Registry publish fails the deploy on a transient 500 #79

The deploy for #78 went red on its last job, publish-mcp-registry, with cannot publish duplicate version.

The job checks whether the version in server.json is already on the official registry before publishing. The registry answered that check with a transient HTTP 500 (it answers 200 now), and the workflow treats anything other than 200 as "not published yet" — so it tried to publish 1.0.1 again and was refused. Nothing was wrong with the listing.

Only a 404 should mean "publish". A 5xx is now retried with backoff, and if it persists the run skips publishing with a warning instead of failing, since the next deploy checks again.

Pages affected:

• MCP Harbor -- the registry whose listing on the official MCP registry this job keeps current.
• MCP endpoint docs -- server directory that agents reach through that listing.