The setup the server wrote with BuildGeminiLiveSetup. Every connection of the session opens with it.
OptionalUpstreamMints the upstream handshake's headers. Called on every upstream open, so a short-lived OAuth token (Gemini Enterprise) is fresh for each connection. Omitted on the Developer API, whose key travels in the upstream URL.
What a GeminiLiveRelayPolicy needs for one relayed session.