Record a fired event's cost into a sink, deduped by its dedupeKey against seen (so at-least-once delivery
can't double-charge). Returns the recorded event, or null when it was a duplicate. seen is the app's dedup
store (an in-memory Set for dev; a durable KV/DO for prod).
Record a fired event's cost into a sink, deduped by its
dedupeKeyagainstseen(so at-least-once delivery can't double-charge). Returns the recorded event, or null when it was a duplicate.seenis the app's dedup store (an in-memory Set for dev; a durable KV/DO for prod).