After the cluster expansion completes the slot migration, the total dbsize value of all shard master nodes is greater than the value before the expansion

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/kvrocks/issues) and found no similar issues.

### Version

v2.9.0

### Minimal reproduce step

1、kvrocks version：v2.9.0
2、init env：3 shard，One master and one slave，Single Node Specifications：8C16G、disk：500GB
3、After expansion：5 shard，One master and one slave，Single Node Specifications：8C16G、disk：500GB
4、Expansion method：Calling the controller interface
5、Phenomenon：
Before expansion： the total dbsize value of all shard master nodes：12484
After expansion：the total dbsize value of all shard master nodes：12489

### What did you expect to see?

The total dbsize of all master nodes is equal before and after expansion

### What did you see instead?

Before expansion： the total dbsize value of all shard master nodes：12484
After expansion：the total dbsize value of all shard master nodes：12489

### Anything Else?

Migration interface for controllers：
POST /api/v1/namespaces/clusters/{cluster name}/migrate
{
"target": 4,
"slot": 1000,
"slot_only": false
}

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Start with the controller migration entry point, POST /api/v1/namespaces/{cluster name}/migrate, and reproduce the three-to-five-shard expansion described for v2.9.0. Trace slot migration and dbsize accounting across the shard masters; done means the total dbsize remains equal before and after expansion.

Written by the indexing model from the issue text.