feat: add --dry-run to aidata download dataset command

[Copilot]

The aidata download dataset command lacked a --dry-run option, making it impossible to preview what would be fetched without triggering actual file writes and media downloads.

Changes

• commands/download.py: Adds @common_args.dry_run decorator and dry_run: bool parameter; skips output directory creation and passes the flag through to the generator.
• generators/coco_voc.py: Adds dry_run: bool = False parameter; after querying and counting records from the Tator API, returns early with a log summary when set — no files written, no media downloaded.
• tests/test_download.py (new): Adds test_download_dataset_dry_run_flag_in_help (no-database) and a database-gated integration test that asserts no images/ or labels/ directories are created.

Example

aidata download dataset \
  --dry-run \
  --config config.yml \
  --token $TATOR_TOKEN \
  --version Baseline
# Logs: "Dry run - would download 1234 records for version ['Baseline'] ..."
# No files written.