Keep datasets files JSON output parseable

[ternaus]

Fixes #1186.

kaggle datasets files --format json printed the pagination token to stdout before the JSON array. The command now sends that token to stderr when an explicit output format is requested, while keeping the table/CSV stdout behavior unchanged.

Tests:

• uv run --with pytest pytest tests/unit/test_cli_datasets.py tests/unit/test_dataset_files.py
• uv run --with black black --check --diff src/kaggle/api/kaggle_api_extended.py tests/unit/test_dataset_files.py

[stevemessick]

/gcbrun

[stevemessick]

Thanks for the contribution! However, this change breaks backward compatibility, so we will not be merging it.