Fix worker CLI tests for optional pool argument

[gopidesupavan]

Adjusted the celery worker CLI unit tests to match the current contract for --pool.

The required-arguments test no longer expects --pool prefork, since that flag is only added when celery.pool is present in config. Added a dedicated test that mocks the config-backed pool path and verifies both the forwarded --pool prefork argument and the corresponding maybe_patch_concurrency() call.

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[gopidesupavan]

closing in favour of https://github.com/apache/airflow/pull/64388/changes