Flink source support detect bucket number change

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

After support rescale bucket in https://github.com/apache/fluss/pull/1554, Flink source should have the ability to detect the bucket change of table or partition.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by reading the rescale-bucket implementation in pull request #1554 and the Flink source integration it affects. Trace how a table or partition's bucket count is observed, then define completion as the Flink source detecting and responding to bucket-number changes.

Written by the indexing model from the issue text.