chore: rename ColumnWiseDatasetBuilder to DatasetBuilder

[andreatgretel]

Context

PR #429 introduced the async task-queue scheduler into ColumnWiseDatasetBuilder. During review, @nabinchha suggested simplifying the class name since it's the only dataset builder and the "column-wise" qualifier is an implementation detail. Deferred from #429 to keep that PR focused on the async integration.

Scope

• Rename the class from ColumnWiseDatasetBuilder to DatasetBuilder in column_wise_builder.py
• Rename the module from column_wise_builder.py to dataset_builder.py (and update all import paths)
• Update all references in source, tests, and scripts

Files affected

• packages/data-designer-engine/src/data_designer/engine/dataset_builders/column_wise_builder.py
• packages/data-designer/src/data_designer/interface/data_designer.py
• packages/data-designer/tests/interface/test_data_designer.py
• packages/data-designer-engine/tests/engine/dataset_builders/test_column_wise_builder.py
• packages/data-designer-engine/tests/engine/dataset_builders/test_async_builder_integration.py
• packages/data-designer-engine/tests/engine/models/test_async_engine_switch.py
• scripts/benchmarks/benchmark_engine_v2.py

Notes

Class rename was tested locally on the andreatgretel/feat/async-builder-integration branch - all 87 affected tests pass. Module file rename was not attempted but should be done together for consistency.