Emit duplicate observability without re-running public result handlers.
Validate sender-controlled dedup identity before callers mutate durable
settlement state. handleWebhook() repeats this check for direct users;
SingleAgentClient invokes it earlier because durable recovery must run
before the public result handler.
Handle incoming webhook payload (both task completions and notifications)
OptionalpreviewHandler?: "canonical" | "legacy"Preserves the originating preview API across async completion.
Async handler class