feat(dag_command.py): change to use bulk clear

[FrankYang0529]

closes: #67484

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Why

airflow dags clear (#66004) loops dag.clear(run_id=...) once per matched run, issuing a separate SELECT + flush for each. A wide --partition-date-* window or broad --partition-key therefore costs N independent transactions.

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How

Replace the per-run loop with a _bulk_clear_runs helper:

• Iterates run_ids in chunks of _RUN_CHUNK_SIZE (500), fetching all matching TIs for each chunk with a single query.
• Passes the entire chunk's TIs to clear_task_instances in one call.

Two new tests:

• test_clears_multiple_runs_in_one_batch: asserts that 3 runs fitting in one chunk result in exactly one clear_task_instances call.
• test_chunks_on_run_boundaries_clears_each_run_once: patches _RUN_CHUNK_SIZE to 2. 3 runs span 2 chunks.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Claude Code with Claude Opus 4.8

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[Lee-W]

mostly good. a few nit

[jason810496]

LGTM overall, thanks!