Register an open GET SSE stream. A fresh per-stream session token is minted
and the listener immediately receives the leading endpoint event
(basic/transports §HTTP with SSE: the server MUST send it "When a client
connects") whose URI carries that token, so a later POST can be correlated
back to exactly this stream. Returns the listener id.
Register an open GET SSE stream. A fresh per-stream session token is minted and the listener immediately receives the leading endpoint event (basic/transports §HTTP with SSE: the server MUST send it "When a client connects") whose URI carries that token, so a later POST can be correlated back to exactly this stream. Returns the listener id.