Frame msg as an SSE event (with a per-stream globally-unique id) and
deliver it on exactly ONE connected stream, honouring the transport's
Multiple Connections rule that the server "MUST send each of its JSON-RPC
messages on only one of the connected streams ... it MUST NOT broadcast the
same message across multiple streams." Listeners are tried in registration
order; one whose write throws (a disconnected client) is dropped and the
next live listener is tried, so the message still lands on a healthy stream
and the channel self-heals. Returns 1 if the message was delivered, or 0
when no live listener could receive it.
Frame msg as an SSE event (with a per-stream globally-unique id) and deliver it on exactly ONE connected stream, honouring the transport's Multiple Connections rule that the server "MUST send each of its JSON-RPC messages on only one of the connected streams ... it MUST NOT broadcast the same message across multiple streams." Listeners are tried in registration order; one whose write throws (a disconnected client) is dropped and the next live listener is tried, so the message still lands on a healthy stream and the channel self-heals. Returns 1 if the message was delivered, or 0 when no live listener could receive it.