The server declares one extension capability, com.example/feature-flags, with
a small settings object via server.registerCapabilities({ extensions: { … } }).
The client connects once per era leg and asserts the entry and its settings are
advertised — by the initialize result on the legacy leg and by
server/discover on the modern leg.
pnpm tsx examples/extension-capabilities/client.ts # modern (server/discover)
pnpm tsx examples/extension-capabilities/client.ts --legacy # 2025 initialize handshake