Process a single JSON-RPC message POSTed to the legacy message endpoint and
route any response back onto the originating client's legacy GET SSE stream as a
message event. sessionId is the per-stream token the client echoed from its
endpoint event, so the response is delivered ONLY to the stream that issued
this POST — never broadcast across concurrently-connected clients. Returns true
(the server accepts every well-formed POST on this transport; a parse failure
still yields a JSON-RPC error response delivered on the stream). A notification
produces no response, so nothing is delivered. Exposed (package-level) so the
two-endpoint flow can be exercised without a live socket.
Process a single JSON-RPC message POSTed to the legacy message endpoint and route any response back onto the originating client's legacy GET SSE stream as a message event. sessionId is the per-stream token the client echoed from its endpoint event, so the response is delivered ONLY to the stream that issued this POST — never broadcast across concurrently-connected clients. Returns true (the server accepts every well-formed POST on this transport; a parse failure still yields a JSON-RPC error response delivered on the stream). A notification produces no response, so nothing is delivered. Exposed (package-level) so the two-endpoint flow can be exercised without a live socket.