[Bug] Shared mutable batchAckIndexList in PopBufferMergeService causes toStoreBits update failure when enablePopBatchAck and appendAckAsync are both enabled

### Before Creating the Bug Report

- [x] I found a bug, not just asking a question, which should be created in [GitHub Discussions](https://github.com/apache/rocketmq/discussions).

- [x] I have searched the [GitHub Issues](https://github.com/apache/rocketmq/issues) and [GitHub Discussions](https://github.com/apache/rocketmq/discussions) of this repository and believe that this is not a duplicate.

- [x] I have confirmed that this bug belongs to the current repository, not other repositories of RocketMQ.

### Runtime platform environment

All platforms

### RocketMQ version

develop branch

### JDK Version

JDK 8+

### Describe the Bug

In `PopBufferMergeService.scan()`, when `enablePopBatchAck` is `true`, the method collects ACK indices into a class-level shared variable `batchAckIndexList`, then passes this same list reference to `putBatchAckToStore()`. Immediately afterwards, it calls `indexList.clear()` in the `finally `block.

When `appendAckAsync `is also `true`, `putBatchAckToStore()` performs an asynchronous write. The async callback `handleBatchAckPutMessageResult `captures the same list reference. By the time the callback executes, the list has already been cleared (or contains stale data from the next loop iteration).

As a result, `pointWrapper.getToStoreBits()` is never updated correctly. This causes `isCkDoneForFinish()` to always return `false`, preventing `PopCheckPointWrapper `objects from ever being removed from the `commitOffsets `queue (an unbounded `LinkedBlockingDeque`). Over time, this leads to unbounded memory growth and eventually OOM.

### Steps to Reproduce

Set `enablePopBatchAck=true` and `appendAckAsync=true`
Run a Pop consumer with moderate traffic
Observe that `commitOffsets `queue size grows indefinitely

### What Did You Expect to See?

`toStoreBits `should be correctly updated after async IO completes. `commitOffsets `queue should be drained normally.

### What Did You See Instead?

`toStoreBits `remains 0. `commitOffsets `queue grows without bound.

### Additional Context

Fix: Pass a defensive copy `new ArrayList<>(indexList)` instead of the shared reference.

Start in PopBufferMergeService.scan() and trace the batchAckIndexList reference into putBatchAckToStore() and handleBatchAckPutMessageResult(). Reproduce with enablePopBatchAck=true and appendAckAsync=true, then verify that async completion updates toStoreBits and allows the commitOffsets queue to drain instead of growing indefinitely.

Written by the indexing model from the issue text.