[AIT-276] feat: enhance object operation handling and test coverage

- Improved handling of skipped operations on ACK (RTO9a3) with detailed comments on echo discard logic.
- Added extensive unit tests for `ObjectsManager`, `LiveMapManager`, and `DefaultLiveCounter` covering edge cases like buffering, tombstoning, and operation deduplication.
- Introduced helper properties for buffered ACKs and object operations in tests.

Author: ttypic

liveobjects/src/main/kotlin/io/ably/lib/objects/ObjectsManager.kt

@@ -223,10 +223,14 @@ internal class ObjectsManager(private val realtimeObjects: DefaultRealtimeObject
         continue
       }
 
-      // RTO9a3 - skip operations already applied on ACK
+      // RTO9a3 - skip operations already applied on ACK (discard without taking any further action).
+      // This check comes before zero-value object creation (RTO9a2a1) so that no zero-value object is
+      // created for an objectId not yet in the pool when the echo is being discarded.
+      // Note: siteTimeserials is NOT updated here intentionally — updating it to the echo's serial would
+      // incorrectly reject older-but-unprocessed operations from the same site that arrive after the echo.
       if (objectMessage.serial != null &&
         realtimeObjects.appliedOnAckSerials.contains(objectMessage.serial)) {
-        Log.d(tag, "RTO9a3: serial ${objectMessage.serial} already applied on ACK; discarding")
+        Log.d(tag, "RTO9a3: serial ${objectMessage.serial} already applied on ACK; discarding echo")
         realtimeObjects.appliedOnAckSerials.remove(objectMessage.serial)
         continue // discard without taking any further action
       }

liveobjects/src/test/kotlin/io/ably/lib/objects/unit/TestHelpers.kt

@@ -73,6 +73,9 @@ internal val ObjectsManager.SyncObjectsDataPool: Map<String, ObjectState>
 internal val ObjectsManager.BufferedObjectOperations: List<ObjectMessage>
   get() = this.getPrivateField("bufferedObjectOperations")
 
+internal val ObjectsManager.BufferedAcks: List<Pair<List<ObjectMessage>, *>>
+  get() = this.getPrivateField("bufferedAcks")
+
 internal var DefaultRealtimeObjects.ObjectsManager: ObjectsManager
   get() = this.getPrivateField("objectsManager")
   set(value) = this.setPrivateField("objectsManager", value)