Forward exist_ok in dataframe_to_mds

[SiluPanda]

Summary

• forward mds_kwargs["exist_ok"] to the initial CloudUploader.get() call
• add an end-to-end Spark regression covering conversion into an existing, non-empty local output directory
• preserve the existing default (False) when callers omit exist_ok

Fixes #871.

Validation

• Fail-first: the new regression raised FileExistsError on the unmodified implementation because CloudUploader.get() received exist_ok=False
• pytest -o addopts='' tests/base/converters/test_dataframe_to_mds.py (22 passed)
• pytest -o addopts='' tests/base/converters/test_dataframe_to_mds.py::TestDataFrameToMDS::test_end_to_end_conversion_local_exist_ok (1 passed)
• Changed-file pre-commit hooks: Ruff, YAPF, isort, syntax/whitespace/license/docformatter checks passed
• Targeted Pyright 1.1.256: 0 errors
• git diff --check

This contribution was implemented with AI assistance and manually reviewed and validated.