fix(config): DraftSnapshotFactory loads only active (unreleased) reservations

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.

src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Validation/DraftSnapshotFactory.cs

@@ -43,6 +43,9 @@ public static class DraftSnapshotFactory
             VirtualTags        = await db.VirtualTags.AsNoTracking().ToListAsync(ct),
             PollGroups         = await db.PollGroups.AsNoTracking().ToListAsync(ct),
             PriorEquipment     = [],
-            ActiveReservations = await db.ExternalIdReservations.AsNoTracking().ToListAsync(ct),
+            ActiveReservations = await db.ExternalIdReservations
+                .AsNoTracking()
+                .Where(r => r.ReleasedAt == null)   // active only — matches DraftSnapshot.ActiveReservations semantics
+                .ToListAsync(ct),
         };
 }