[python] Inherit watermark in PyPaimon commits

[JingsongLi]

What

Make PyPaimon commits inherit the watermark from the latest snapshot.

Why

Paimon snapshots may omit the watermark only when neither the current commit nor the previous snapshot has one. PyPaimon currently creates every new snapshot without copying the latest watermark, which breaks watermark continuity in mixed Java/Python write histories and can invalidate watermark-based time travel.

Impact

After this change, a PyPaimon commit following a watermark-bearing Java/Flink snapshot preserves that watermark. Tables whose history has never carried a watermark continue to write snapshots without one.

Tests

• python3 -m pytest pypaimon/tests/file_store_commit_test.py -q
• python3 -m pytest pypaimon/tests/reader_append_only_test.py::AoReaderTest::test_commit_retry_filter -q
• flake8 --config=./dev/cfg.ini pypaimon/write/file_store_commit.py pypaimon/tests/file_store_commit_test.py
• git diff --check

[leaves12138]

Reviewed the watermark inheritance semantics against the Java commit path and the PyPaimon retry flow. The change correctly preserves the latest snapshot watermark while keeping it absent for histories without one. Targeted commit tests passed.