fix(unreal): suppress spurious OnInsert on overlapping subscription refcount bump

ClientCache::ApplyDiff phase 2 unconditionally appended every incoming
row to Diff.Inserts, even when the row was already cached and only the
refcount needed to bump. BroadcastDiff then fired OnInsert for every
entry, so any table subscribed via two overlapping queries (e.g. a
global SELECT plus a per-row WHERE) re-fired its insert handler on
every later subscription apply.

Mirror the delete path: only emit a Diff.Inserts entry when the
row-bytes key transitions from absent to refcount=1. Real updates are
unaffected — update bytes differ from old bytes, take the !Entry path,
and DeriveUpdatesByPrimaryKey still reconciles them into OnUpdate.

Author: SamuelE-Arvika

sdks/unreal/src/SpacetimeDbSdk/Source/SpacetimeDbSdk/Public/DBCache/ClientCache.h

@@ -120,20 +120,24 @@ class UClientCache
 
             TSharedPtr<RowType> NewRow = MakeShared<RowType>(Row);
 
-            // This is a new row or an update to an existing row that was not deleted.
             FRowEntry<RowType>* Entry = Table->Entries.Find(Key);
             if (!Entry)
             {
-                // True insert
+                // True insert — these row-bytes are not cached yet. Either a
+                // genuinely new row, or the insert half of an update (the
+                // paired delete of different bytes is tracked in phase 1/3;
+                // DeriveUpdatesByPrimaryKey reconciles them by PK afterward).
                 Table->Entries.Add(Key, FRowEntry<RowType>{NewRow, 1});
+                Diff.Inserts.Add(Key, *NewRow);
             }
             else
             {
-                // True update
+                // Refcount bump — an overlapping subscription brought an
+                // identical row already in cache. Mirror the delete path
+                // (which only emits Diff.Deletes on refcount == 0) by not
+                // emitting a spurious Diff.Inserts entry here.
                 Table->Entries.Add(Key, FRowEntry<RowType>{NewRow, Entry->RefCount + 1});
             }
-
-            Diff.Inserts.Add(Key, *NewRow);
         }
 
         // Phase 3: Finalize Deletes and Update Indices