fix(unreal): suppress spurious OnInsert on overlapping subscription refcount bump (#4903)

# Description of Changes

`UClientCache::ApplyDiff` (`sdks/unreal/.../DBCache/ClientCache.h`) has
an asymmetry between its insert and delete paths.

**Phase 1 (deletes)** correctly only emits a `Diff.Deletes` entry when
the row's refcount transitions to 0 — overlapping subscriptions just
decrement.

**Phase 2 (inserts)** always appends to `Diff.Inserts`, regardless of
whether the row was already cached:

```cpp
// before
FRowEntry<RowType>* Entry = Table->Entries.Find(Key);
if (!Entry) { /* refcount = 1 */ }
else        { /* refcount + 1 */ }
Diff.Inserts.Add(Key, *NewRow);   // ← fires for both branches
```

`BroadcastDiff` then fires `OnInsert` for every `Diff.Inserts` entry, so
any table subscribed by two overlapping queries (e.g. a global `SELECT *
FROM t` plus a per-row `WHERE id = ...`) re-fires its insert handler on
every later subscription apply — once per cached row, every time. Game
code that does work in `OnInsert` (positioning, spawning, snapping to
terrain) re-runs and clobbers state that was meant to be set once.

The intent is documented in `RowEntry.h`: *"Wrapper storing a row value
with a reference count used by overlapping subscriptions."* Phase 1
follows that design; phase 2 doesn't.

### Fix

Move the `Diff.Inserts.Add` into the `!Entry` branch only, so it fires
only on the absent → refcount=1 transition:

```cpp
if (!Entry) {
    Table->Entries.Add(Key, FRowEntry<RowType>{NewRow, 1});
    Diff.Inserts.Add(Key, *NewRow);
}
else {
    Table->Entries.Add(Key, FRowEntry<RowType>{NewRow, Entry->RefCount + 1});
}
```

### Why real updates still work

Cache keys are BSATN row-bytes, not primary keys. A real update arrives
as a `(delete old_bytes, insert new_bytes)` pair where `old_bytes ≠
new_bytes` — so the insert side still takes the `!Entry` branch and gets
a `Diff.Inserts` entry. `FTableAppliedDiff::DeriveUpdatesByPrimaryKey`
then pairs the delete and insert by PK into
`UpdateInserts`/`UpdateDeletes`, and `OnUpdate` (not `OnInsert`) fires,
exactly as today.

Edge cases:

| Scenario | Phase 2 branch | Result | Correct? |
|---|---|---|---|
| New row | `!Entry` | `OnInsert` | ✓ |
| Real update (different bytes) | `!Entry` | `OnInsert`+`OnDelete`
reconciled to `OnUpdate` by PK | ✓ |
| Overlapping sub re-delivers cached row | `else` | refcount bump, no
event | ✓ (was broken — fired duplicate `OnInsert`) |
| Trivial update (identical bytes) | `else` | refcount bump | irrelevant
— server diffs identical rows away before emitting |

# API and ABI breaking changes

None. Purely internal cache bookkeeping. Existing
`OnInsert`/`OnDelete`/`OnUpdate` semantics are preserved for all
non-overlapping cases; the only behavior change is that overlapping
subscriptions stop emitting duplicate `OnInsert` events for
already-cached rows — which matches the documented `RowEntry` refcount
contract.

# Expected complexity level and risk

**1.** Two lines moved into a branch; comments updated. Mirrors logic
already present and known-correct in phase 1.

# Testing

Reproduced and validated downstream in an Unreal project. The repro
setup is straightforward to replicate against any module:

1. A table `t` with ~150 rows.
2. A global subscription `SELECT * FROM t`, applied first.
3. A diagnostic actor that binds `t.OnInsert`, then every 10s submits a
new overlapping subscription (e.g. `SELECT * FROM t` again, or any
`SELECT * FROM t WHERE 'id' = X` covering already-cached rows) and
counts the `OnInsert` events that arrive in that round.

Expected: round 1 fires once per row that is *new to the cache*;
subsequent rounds against already-cached rows fire 0.

Observed (~161 rows cached after initial load):

```
                              Pre-fix     Post-fix
Global sub (empty → 161)      161         161        ← genuine inserts; unchanged
Round 1 (overlapping)         134         0          ← 134 cached dupes
Round 2 (overlapping)         134         0
Round 3 (overlapping)         134         0
Round 4–6 (overlapping)       134 each    0 each
```

The genuine "empty cache → 161 entries" wave on the initial global
subscription is unaffected — same `OnInsert` count both pre- and
post-fix. Only the duplicate fires from later overlapping subscriptions
on already-cached rows are eliminated. `OnUpdate` still fires correctly
when underlying rows actually change.

- [x] Reviewer: confirm an existing real-update (different bytes, same
PK) test still produces `OnUpdate` and not `OnInsert`+`OnDelete`.
- [x] Reviewer: confirm any test that relies on `OnInsert` firing on
every subscription apply (rather than only on cache 0→1 transition) is
not present — if it exists, it was relying on the bug.

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