fix: handle optional 'description' in example registry server

[x0cipher]

Hit this while poking at the conformance demo: description is optional (§4.2), but the example registry assumes it's always there, so a valid catalog that omits it just 500s on /search. Small fix, but the demo never covered that path so I added a case for it too.

What changed

• registry-server.py: search matches on entry.get("description", "") instead of entry["description"]
• added a description-less entry to the mock catalog + examples/ai-catalog.json (kept in sync) so run-conformance-demo actually exercises the optional path

Before / after

• before: manifest validation passes, but the /search probe dies with Remote end closed connection
• after: demo's green end to end

[mindpower]

Thank you @x0cipher for fixing this!