feat(python): add abort method to TableCommit for write rollback

[hechao-ustc]

Purpose

The Python binding is missing write rollback support. The core Rust TableCommit has an abort() method and the C binding already exposes it via paimon_table_commit_abort, but Python users have no way to clean up data files after a failed write. This PR adds abort() to the Python binding so callers can delete newly written files (data, changelog, index) before a commit is persisted.

Brief change log

• Extract message collection and validation into collect_and_validate_messages helper, shared by commit() and abort()
• Add PyTableCommit::abort() — calls through to TableCommit::abort(), best-effort file deletion (storage errors silently ignored)
• Add 5 unit tests covering the abort lifecycle and all validation paths

Tests

New abort-specific tests in bindings/python/tests/test_write.py:

Test	Covers
test_abort_cleans_up_written_data	Write → abort → table is empty
test_abort_empty_messages_noop	Abort with no messages succeeds
test_abort_non_message_raises_typeerror	Non-message input raises TypeError
test_abort_cross_table_messages_raises	Messages from a different table rejected (ValueError)
test_abort_different_builder_same_table_raises	Messages from a different WriteBuilder rejected (ValueError)

Run with:
cd bindings/python
.venv/bin/maturin develop --release
.venv/bin/python -m pytest tests/test_write.py -v -k abort

API and Format

New method on TableCommit:

table.new_write_builder().new_commit().abort(messages)

• messages: a sequence of CommitMessage objects returned by prepare_commit()
• Returns None on success
• Raises TypeError if messages is not iterable or contains non-CommitMessage items
• Raises ValueError if messages were prepared for a different table or by a different WriteBuilder
• Deletion is best-effort — no error is raised for missing files or storage failures
• Empty messages is a no-op

Documentation