feat: support dynamic-bucket partial-update

[QuakeWang]

Purpose

Linked issue: follow-up to #256

This PR adds basic merge-engine=partial-update support for primary-key tables using bucket=-1 dynamic bucket mode.

It extends the existing fixed-bucket partial-update support to non-cross-partition dynamic bucket tables, including partitioned tables whose primary keys include partition fields. Cross-partition dynamic bucket partial-update remains unsupported and fails fast.

Brief change log

• Allow merge-engine=partial-update for non-cross-partition bucket=-1 PK tables.
• Restore existing HASH index entries before assigning buckets, so updates to existing keys stay in their original buckets.
• Keep merge-engine=partial-update with cross-partition dynamic bucket explicitly unsupported.
• Reject partition-only primary keys, which would leave only one record per partition.
• Add DataFusion integration tests for unpartitioned, partitioned, and existing-key dynamic bucket partial-update cases.

Tests

API and Format

This PR uses the existing dynamic bucket HASH index format and does not introduce a new storage format.

Documentation

[JingsongLi]

+1