Streamable HTTP: opt: don't store events for single message responses #577
Description
Related to #576, the streamable server always stores events in the event store when responding to streamable requests.
...but if there is only one event on the stream (the response), there is nothing to replay. We should avoid storing these events that will never be required.
Implementing this logic should also enable responding with application/json when there is only a single message (see #575).