Pre-7/28 launch: probe top servers for graceful negotiation-by-failure

### Where was this agreed?

https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/2905

### Description

New 7/28 clients will fail their first request against not-yet-upgraded servers (negotiation-by-failure). Before launch, test that the top ~50 servers handle this gracefully and document the expected failure pattern for client authors.