fix(sdk): omit since on SSE reconnect (#2606)

## Summary
- Stop sending body `since` on SSE (re)connect in
`ProtocolSseTransportAdapter`.
- Protocol `seq` is connection-scoped; a new POST re-numbers Redis
history from 1, so a prior session's `since` (e.g. `146`) dropped the
entire replay and left only heartbeats after QUIC/idle drops.
- Clients continue to deduplicate via durable `event_id`. A durable
cursor can replace this later.