[spark] Support delete records for pk table

### Search before asking

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

### Motivation

As title.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

The issue names Spark and primary-key tables but provides no files, tests, or implementation entry point. Start by locating the Spark integration and primary-key table handling, then determine the expected delete behavior and add focused coverage showing that records can be deleted successfully.

Written by the indexing model from the issue text.