feat: support position delete writer

[chenzl25]

Position delete writer overhaul:

• Replaced the stale stub with a real PositionDeleteFileWriter built on RollingFileWriter, so positional deletes are written as proper Parquet files with correct Arrow schema, partition metadata, and
  CurrentFileStatus.
• Added a small Arrow batch builder helper plus partition-aware close logic to tag returned DataFiles as PositionDeletes.
• Introduced async unit tests for both unpartitioned and partitioned scenarios that round-trip through a Parquet writer and reuse check_parquet_data_file for verification.