feat: wire build_preview() into async scheduler when DATA_DESIGNER_ASYNC_ENGINE=1

[andreatgretel]

Context

When DATA_DESIGNER_ASYNC_ENGINE=1 is set, build() correctly routes through _build_async() and the AsyncTaskScheduler. However, build_preview() still goes through the sequential _run_batch() path - columns are processed one at a time, waiting for all records to complete before starting the next column.

For a recipe pipeline with 7 columns and 3 records, preview took ~52s sequentially. Independent columns (e.g., two recipe_idea columns on different providers) could run concurrently, and downstream columns could start as soon as their per-row dependencies are met.

Proposed approach

Reuse _build_async() with preview-specific behavior:

• Single row group, no disk checkpoints
• Return in-memory DataFrame instead of writing to disk
• Skip metadata writes

~50-100 lines, mostly conditional logic around checkpointing. The async scheduler itself needs no changes.

Related

• PR feat: wire async task-queue scheduler into ColumnWiseDatasetBuilder #429 (async builder integration)
• Reported by @nabinchha in PR #429 comment