fix: compact blob v2 datasets via rewrite fallback

[everySympathy]

Summary

Adds a BlobV2-safe compaction fallback for daft-lance. When a dataset contains BlobV2 columns, compaction rewrites the visible rows instead of using the existing path that cannot safely handle BlobV2 columns.

Changes

• Detects BlobV2 columns before running compaction.
• Uses take_blobs to materialize BlobV2 values for visible rows.
• Rewrites the dataset through a fallback path that preserves logical row contents.
• Adds tests covering BlobV2 compaction behavior.

Testing

• uv run --python /usr/bin/python3.11 pytest tests/io/lancedb/test_lancedb_compaction.py -q
  • Result: 6 passed
• Lint/type checks for touched files passed locally.

Notes / Risks

• This is intentionally a rewrite fallback, not native in-place BlobV2 compaction.
• External/reference blobs are materialized into managed bytes by this fallback.
• Native BlobV2-aware compaction can be added later when Lance exposes a safer direct path.