Stateless StreamableHTTPServerTransport: per-request requirement is invisible through the Node wrapper (500 with empty body)

Problem

The WebStandard core throws a clear assertion when a stateless transport is reused:

Stateless transport cannot be reused across requests. Create a new transport per request.

The Node wrapper (StreamableHTTPServerTransport via @hono/node-server) converts that assertion into a bare 500 with an empty body — no onerror, no rejection. Combined with Client.connect() unconditionally sending notifications/initialized, a stock official-client <-> official-stateless-server pair fails with an opaque 500, giving no signal that the per-request assembly pattern is required.

Suggestion

1. Surface the assertion through onerror (or reject from handleRequest) so the failure is observable and debuggable.
2. Document the per-request pattern ("create a new transport per request") prominently in the stateless guide — it is currently invisible through the Node wrapper.

Context

Observed while integrating the 2026-07-28 stateless core (@modelcontextprotocol/sdk 1.30.0) as an official protocol face. Our workaround (per-request assembly + a notification-dropping client transport) is documented in our project's known-issues log.

Start with the Node wrapper's StreamableHTTPServerTransport and its handleRequest path, then compare the WebStandard assertion with the Node behavior. Check Client.connect() and the stateless guide for the per-request pattern. Done means the reuse failure is observable through onerror or a rejected request, and the guide clearly documents creating a new transport per request.