SubscribeAsync left the SDK Subscription on the session (server-side when
CreateAsync succeeded, item handlers attached, tracked by nothing) when
CreateAsync/CreateItemsAsync threw, so the host's per-tick reconcile retry
accumulated one live subscription per retry. Mirror the alarm path: detach
the item handlers, bounded RemoveSubscriptionAsync, rethrow.
Refs #743
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty