Filter ExternalIdReservations to WHERE ReleasedAt IS NULL so
DraftSnapshot.ActiveReservations matches its documented semantics and
ValidateReservationPreflight cannot emit spurious BadDuplicateExternalIdentifier
errors from already-released rows. Adds a focused unit test seeding one active
and one released reservation and asserting only the active row is returned.
Joseph Doherty
46aba992c5