Fix Beam async hook launching pipelines through a shell

[r3wretrhy]

BeamAsyncHook started pipeline processes by joining the argv list with POSIX shlex quoting and running the result through a shell. The sync hook already uses subprocess.Popen with shell=False and the original list. The async path could therefore split interpreter or pipeline paths that contain spaces, especially on Windows where POSIX quoting is not what the shell expects.

This runs _beam_version and run_beam_command_async with asyncio.create_subprocess_exec so each argument stays one argv entry. Missing interpreters still raise AirflowException. Tests assert exec is used (not shell) and that an argument containing spaces survives a live process.

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

• Yes (Grok)

Generated-by: Grok following the guidelines

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• Tests run locally: python verification of run_beam_command_async with a spaced argument, exec-vs-shell construction, and _beam_version error wrapping. Full provider suite via CI: providers/apache/beam/tests/unit/apache/beam/hooks/test_beam.py.

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[potiuk]

this must wait for #66952