Tolerate a non JSON-RPC response body for notifications

[orthodoX]

Problem

The MCP spec says a server answers a notification (a message with no id) with 202 Accepted and no body, which the transport handles. Some servers (e.g. https://mcp.trainual.com) instead return 200 with body: { ok: true } that is not a JSON-RPC envelope.

handle_success_response validates every 200 JSON body as an envelope, so the notifications/initialized step raises TransportError: Invalid JSON response: Invalid JSON-RPC envelope, killing the handshake right after a successful initialize.

Fix

When the message was a notification (request_id is nil), skip the JSON body entirely, nothing waits on it. Requests keep the strict envelope validation. The session id header is still captured, since that happens in handle_response before the body is read.

One existing spec asserted a Result was returned for a notification response; updated it to expect nil while confirming the session id is still captured.