0380 — The datastore interfaces are deleted in favor of the concrete types

Context. The consumer’s Datastore[Message] interface had the same disease that already killed the producer’s during the batching work: NewConsumerDatastore returns the concrete type and NewMessageConsumer constructs it internally from a concrete *datastore.PostgresDatastore, so nothing could ever substitute an alternative implementation. An unsubstitutable interface is dead API surface, not a seam. Options were: fix the coupling plus the pgtype.UUID token leak into a real seam, thin it to what testing needs, or collapse it.

Decision. Collapse — removed for now. Deleted Datastore[Message] (its interface-only doc comments merged onto the concrete methods: Commit’s initial-backoff semantics, PartialCommit’s lease-narrowing, the disableDeliveryLog notes) and the equally dead one-method Consumer[Message] interface, same fate as the producer’s Producer[Message]. The concrete type is exported as ConsumerDatastore[Message] so labs can name it in helper signatures.

Consequences. Exporting the concrete type also deleted routinglab’s bindable workaround interface, whose whole premise was the concrete type being unexported. If a real seam is wanted after the cleanup, it gets designed as one then: substitution actually possible, token/UUID types not leaking pgx, thinned to what a second implementation or tests genuinely need.