fix: use value-only heap in OnDispose(Deleted) to prevent key-overflow double-subtract

OnDispose(Deleted) now uses GetValueHeapMemorySize() instead of
CalculateHeapMemorySize(). This prevents double-subtracting key overflow
bytes for immutable-region deletes where:
1. OnDispose(Deleted) fires on the source (subtracts full heap including key)
2. ClearHeapFields(clearKey=false) keeps key alive for chain traversal
3. Source is sealed (not tombstoned) — OnEvict visits it later
4. CalculateHeapMemorySize returns key overflow → subtracted again

With the fix:
- OnDispose(Deleted) subtracts value-only heap (value overflow + value object)
- For mutable deletes: tombstoned → OnEvict skips → key overflow is a bounded
  phantom freed by GC when the page is freed (same as pre-IRecordTriggers)
- For immutable deletes: sealed → OnEvict visits → subtracts key overflow once
- Net: no double-decrement, bounded key-overflow phantom on mutable deletes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: badrishc

libs/server/Storage/Functions/GarnetRecordTriggers.cs

@@ -56,11 +56,14 @@ public void OnDispose(ref LogRecord logRecord, DisposeReason reason)
 
             if (reason == DisposeReason.Deleted)
             {
-                // Decrement the full heap contribution of the dying record (overflow key/value bytes
-                // and/or value-object heap). Tombstone is NOT yet set when this fires — Tsavorite sets
-                // it AFTER OnDispose returns — so CalculateHeapMemorySize returns the correct non-zero
-                // value. ClearHeapFields then physically frees the heap fields from the ObjectIdMap.
-                var size = MemoryUtils.CalculateHeapMemorySize(in logRecord);
+                // Decrement only the VALUE-side heap (value overflow bytes or value-object heap).
+                // Key overflow is NOT subtracted here because ClearHeapFields(clearKey=false) keeps the
+                // key alive for hash-chain traversal. Key overflow is handled separately:
+                //  - Mutable delete: record is tombstoned after OnDispose → OnEvict skips it → key overflow
+                //    is a bounded phantom (freed by GC when the page is freed, same as pre-IRecordTriggers).
+                //  - Immutable delete: source is sealed (not tombstoned) → OnEvict visits it and subtracts
+                //    the remaining key overflow via CalculateHeapMemorySize.
+                var size = logRecord.GetValueHeapMemorySize();
                 if (size != 0)
                     cacheSizeTracker.AddHeapSize(-size);
             }