refactor(datasets): count the same id field the insert binds

Drop the fall back to item.id() when datasetItemId is null. It looked
defensive but was worse than nothing: the INSERT ten lines below binds
item.datasetItemId().toString() with no null check, so a null cannot
survive to be counted either way, and every caller normalizes
datasetItemId before reaching here. All the fallback added was a second,
divergent definition of item identity in the one place that must agree
with what the INSERT writes.

Addresses review feedback on #7966.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: petros-double-test1

apps/opik-backend/src/main/java/com/comet/opik/domain/DatasetItemVersionDAO.java

@@ -3502,10 +3502,11 @@ public Mono<Long> insertItems(@NonNull UUID datasetId, @NonNull UUID newVersionI
         //
         // Count DISTINCT stable ids, not rows handed in (OPIK-7891): reads collapse a repeated
         // dataset_item_id to one row via LIMIT 1 BY, so counting the raw list inflates every
-        // version total derived from this value. Callers set datasetItemId before inserting;
-        // fall back to id so an unnormalized item still counts as itself rather than as null.
+        // version total derived from this value. Counting the same field the INSERT binds below
+        // keeps one definition of identity -- every caller normalizes datasetItemId first, and a
+        // null would fail at the bind regardless, so there is nothing to fall back to.
         long itemCount = items.stream()
-                .map(item -> item.datasetItemId() != null ? item.datasetItemId() : item.id())
+                .map(DatasetItem::datasetItemId)
                 .distinct()
                 .count();