The earlier waiting_since fallback in episode_key_for also matched at fire time,
so an agent reply (which clears waiting_since) no longer ended the episode and the
rule could fire anyway. Apply the created_at fallback only at arm time
(arm_episode_key_for); keep episode_key_for strict so a nil waiting_since at fire
time still means the agent replied and the episode is over.
Tanmay Deep Sharma
7a8903051d