Fix async Beam subprocess launch

[avzamelek]

What

Remove the redundant shell=True argument from
BeamAsyncHook.run_beam_command_async.

asyncio.create_subprocess_shell() already starts the subprocess through a
shell. Passing shell=True again is forwarded to subprocess.Popen, where it
conflicts with the value supplied internally and raises a TypeError.

Test coverage

Add a focused async unit test that verifies the subprocess launcher is called
without the duplicate shell keyword.

Testing

• git diff --check

• Attempted the focused regression test with Breeze:

  breeze run pytest providers/apache/beam/tests/unit/apache/beam/hooks/test_beam.py::TestBeamAsyncHook::test_run_beam_command_async_does_not_pass_shell_keyword -q

• Breeze test setup was completed successfully.

• The targeted test was run locally using Docker and passed successfully.

Scope

This is a bug fix for the Apache Beam provider's async execution path. It does
not change command construction or shell-escaping behavior.

Development assistance

Claude Code Fable 5 was used as a development assistant during investigation and
implementation. The final diff was reviewed by human before submission.

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