GraphQL G3: subscriptions over SSE (graphql-sse) #1585

Part of #1581. Depends on G1 and G2.

graphql-sse distinct-connections mode, on core SSE (EventStreamRepresentation, EventStreamReader):

• Server: a subscription with Accept: text/event-stream streams next events and a final complete. One item requested at a time; client disconnect cancels the Publisher. Queries and mutations over SSE send one next then complete.
• Limits: 64 concurrent subscriptions per adapter by default; bounded bytes per subscription.
• Client: subscribe(...) returns a Publisher<ExecutionResult> honoring request(n); cancellation aborts the request; stop() cancels open subscriptions.
• Single-connection mode stays out of scope.