everything-server: subscriptions/listen responds with newline-delimited JSON, not SSE

[dhruvtv]

• The subscriptions/listen handler in examples/servers/typescript/everything-server.ts sets Content-Type: application/json and writes JSON.stringify(frame) + '\n' for the acknowledgment and each notification.
• The Streamable HTTP transport (2026-07-28) says the listen response is an SSE stream, so spec-conforming clients can't read it.
• Suggested fix: Content-Type: text/event-stream, and write each frame as data: {...}\n\n.