[python][ray] Add time-based resumable commits to write_paimon

[XiaoHongbo-Hope]

Purpose

Allow long-running Ray primary-key updates to commit periodically and resume after a driver restart.

Changes

• add PaimonOffsetSource with a pinned source snapshot and stable offsets;
• add commit_mode="incremental", operation_id, commit_interval_seconds, and update_cols to write_paimon;
• store source progress in the same target snapshot as each data commit;
• retain the source snapshot and latest checkpoint with internal tags;
• reject concurrent target commits and schema changes;
• add delete_write_paimon_checkpoint for completed operations.

The default atomic path is unchanged. Incremental mode currently supports fixed-bucket primary-key targets using merge-engine=partial-update; sequence fields and cross-partition updates are not supported.

Tests

• real Ray E2E: time-triggered commits, restart recovery, empty results, inserts, concurrent commits, and schema changes;
• existing Ray write and commit regression suites;
• project flake8 and git diff --check.