Add async dataset runner for concurrent task execution

[MagellaX]

Adds run_dataset_async to fan out run_single_task calls with an asyncio concurrency cap.
Supports group_size runs per task and returns a flat list of Trace results.
Includes focused tests for the async runner’s behavior and grouping.

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Note

Medium Risk
Introduces new concurrency and error-handling behavior around task execution; failures are now captured per-task and returned as error Traces, so ordering/throughput and partial-failure semantics should be validated.

Overview
Adds run_dataset_async as a new public API to execute datasets by concurrently fanning out run_single_task calls with an asyncio semaphore cap, optional group_size repeats, and deterministic result ordering.

Updates the datasets package exports to include run_dataset_async, and adds tests covering empty input validation and group_size repetition/trace naming.

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[MagellaX]

any thoughts bro?? @lorenss-m