Honor identity sort orders for Arrow table writes

[adonm]

Related: #271

What changes?

Honors table sort orders for materialized pyarrow.Table writes when every sort field uses an identity transform and one consistent null placement:

• sorts unpartitioned tables before bin packing
• sorts each final Iceberg partition independently before bin packing
• carries the table sort-order ID through WriteTask
• writes the truthful DataFile.sort_order_id

Unsupported transforms, nested/missing fields, mixed null placement, and streaming RecordBatchReader writes preserve current behavior: data is not claimed as sorted and the file sort-order ID remains null. A warning explains why.

Why?

PyIceberg accepts table sort metadata but currently writes unsorted files and hard-codes sort_order_id=None. This prevents readers from safely using sort-order-aware pruning and leaves manifest metadata inconsistent with users' write intent.

Testing

• Unit test verifies physical ascending order, nulls-last behavior, and returned sort-order ID
• Integration test verifies scan order and manifest sort_order_id
• Local InMemoryCatalog append/scan/manifest round trip passed
• PYTHONPATH=. uv run pytest tests/io/test_pyarrow.py -k sort_table_for_identity_sort_order -q — passed
• ruff check on changed files — passed
• git diff --check — passed