[BugFix] Preserve media access args for stage configs

[xiaohajiayou]

Purpose

Fix #2943

This fixes --allowed-local-media-path not taking effect for Qwen3-TTS Base requests when the service is started with --stage-configs-path.

The root cause was that AsyncOmniEngine._strip_single_engine_args() stripped inherited parent EngineArgs fields before stage config resolution, which unintentionally dropped:

• allowed_local_media_path
• allowed_media_domains

As a result, the per-stage model config never saw those media access settings, and local file://... reference audio was rejected even though --allowed-local-media-path had been provided.

Changes

• keep allowed_local_media_path and allowed_media_domains in _PARENT_ARGS_KEEP
• add a regression test covering preservation of these media access control args through parent arg stripping

Test Plan

• Add a regression test to make sure allowed_local_media_path and allowed_media_domains are not stripped when using --stage-configs-path.
• Verify the fix locally by starting a Qwen3-TTS Base service and sending a request with a local file:// reference audio.

Test Result

• Ran python -m pytest tests/test_config_factory.py -k allowed_media_access_controls locally, and it passed.
• Also verified the fix locally with a real Qwen3-TTS Base service started with --stage-configs-path and --allowed-local-media-path /root/vllm-omni.
• Sent a /v1/audio/speech request with ref_audio=file:///root/vllm-omni/issue2943_ref.wav, got 200 OK, and the generated audio was written to /root/vllm-omni/issue2943_output.wav.

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[xiaohajiayou]

Could you please take a look at whether this PR can be merged? Before the YAML refactor and migration are completed, this PR is still needed.
@lishunyang12