Honor identity sort orders for Arrow table writes

[adonm]

Related: #271 and #3848

Rationale for this change

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.

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.

Are these changes tested?

• Unit tests cover ascending/descending ordering, nulls-first/nulls-last placement, the returned sort-order ID, and the partitioned write path
• Integration test verifies scan order and manifest sort_order_id against a REST catalog
• PYTHONPATH=. uv run pytest tests/io/test_pyarrow.py -k "sort_table_for_identity_sort_order or write_sorted_data_files_per_partition" -q — passed
• ruff check on changed files — passed
• git diff --check — passed

Are there any user-facing changes?

Yes. Tables with an identity-transform sort order now write physically sorted data files carrying the truthful sort_order_id; previously all files were written unsorted with a null sort-order ID. Unsupported sort orders keep the previous behavior. Changelog label requested.

Tooling note: developed with assistance from DS v4 Pro. I reviewed and verified the changes.

[adonm]

Synced with latest main (now 13 commits ahead of the original base, including the pyarrow 24→25 upgrade).

pyarrow 25 deprecated the SortOptions-level null_placement argument (per-key null placement was added in Arrow 25 / GH-46926), and the project's pytest config promotes warnings to errors, so the original pc.sort_indices(..., null_placement=...) call started failing against the new base. Fixed with a version-aware call and tightened the annotations so strict mypy stays clean (zero new errors vs main).

Verified locally: unit test, docker-based REST catalog integration test (test_write_identity_sort_order), ruff, and mypy.