Session replay: early chunks saved under the wrong session #537

If you send events from another pipeline with a stable __deviceId, for example to match session recordings to a session tracked through a CDP, the first replay chunks end up under a different session from the events

Replay chunks recorded before the first track response have no sessionId, so the SDK queues them. When the session arrives, flush() sends them as they are (buildFlushPayload returns replay payloads unchanged), and the server falls back to the session it works out from IP and user agent. Usually that's the same session. With __deviceId set, it isn't. The early chunks, including the full snapshot, are stored under a second session, and the replay of the real session starts without a snapshot

Repro: enable session replay, send a track with properties.__deviceId set to a stable id, then check session_replay_chunks. The early chunks have a different session_id from the event