fix: streaming response due to high number of redis pub/sub channels, better handling of connection issues (#6114)

* fix: unsubscribe redis pubsub when connection closes during prediction or after successful response

* update: add heartbeat to keep connection alive and start/stop heartbeat on server start/stop

* refactor: redis pub/sub methods, add unsubscribe, await subscribe

* fix: redundant unsubscribe

* fix: use map to store clients list

* fix: update how thinking event is handled in redis pub/sub and sse

* fix: review feedback