Support restricting vector search to index shards

### Search before asking

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

### Motivation

Support restricting vector search to a specific set of index shards.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

No files or tests are named. Start by locating the vector-search entry point and the index-shard selection logic, then determine how a caller would provide a restricted shard set. Done means vector search accepts that restriction and tests demonstrate searches limited to the requested index shards.

Written by the indexing model from the issue text.