test: use anyio in async regression slice

[Thump604]

Derived from the current-main restack work for #226, but limited to the pushable test-side cleanup.

Changes:

• convert the current async regression slice to @pytest.mark.anyio
• replace asyncio.get_event_loop().run_until_complete(...) with asyncio.run(...) in the API key tests

Why split this out:

• I also prepared the Python 3.13 CI matrix/workflow changes locally, but this auth context cannot push workflow-file updates (.github/workflows/ci.yml) because GitHub requires workflow scope for OAuth-app pushes.
• This PR lands the tested Python-side compatibility cleanup now, without waiting on that auth limitation.

Local validation:

• PYTHONPATH=/Users/David/code/vllm-mlx-pr226-async-tests /opt/ai-runtime/venv-live/bin/python -m pytest /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_server.py -q -k "verify_api_key or reasoning_stream"
• PYTHONPATH=/Users/David/code/vllm-mlx-pr226-async-tests /opt/ai-runtime/venv-live/bin/python -m pytest /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_batching.py -q -k TestEngineAsync
• PYTHONPATH=/Users/David/code/vllm-mlx-pr226-async-tests /opt/ai-runtime/venv-live/bin/python -m pytest /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_streaming_latency.py -q -k test_output_collector
• PYTHONPATH=/Users/David/code/vllm-mlx-pr226-async-tests /opt/ai-runtime/venv-live/bin/python -m black --check --fast /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_batching.py /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_batching_deterministic.py /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_continuous_batching.py /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_server.py /Users/David/code/vllm-mlx-pr226-async-tests/tests/test_streaming_latency.py

Related to #226.

[janhilgard]

LGTM. Clean test-only changes:

• asyncio → anyio markers — broader backend compatibility, needed for Python 3.13+
• get_event_loop().run_until_complete() → asyncio.run() — the former is deprecated since 3.10 and removed from default behavior in 3.12+

No production code touched.