why lookup join condition must include all primary keys of the dimension table

### Search before asking

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

### Motivation

now lookup join condition must include all primary keys of the dimension table,Is there any particular reason for this? Sometimes my dimension tables may be used in different scenarios, and the condition fields are flexible

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

The issue names no files, tests, or entry points. Start by locating where lookup join conditions are validated against dimension-table primary keys, then determine the rationale for requiring all keys and whether partial-key conditions are supported. Done means an agreed behavior, documented rationale, and tests covering the decision.

Written by the indexing model from the issue text.