Add inline vLLM inference provider to regression tests and fix regressions

[frreiss]

What does this PR do?

This PR adds the inline vLLM inference provider to the regression tests for inference providers. The PR also fixes some regressions in that inference provider in order to make the tests pass.

Test Plan

Command to run the new tests (from root of project):

pytest \
    -vvv \
    llama_stack/providers/tests/inference/test_text_inference.py \
    --providers inference=vllm \
    --inference-model meta-llama/Llama-3.2-3B-Instruct \

Output of the above command after these changes:

/mnt/datadisk1/freiss/llama/env/lib/python3.12/site-packages/pytest_asyncio/plugin.py:207: PytestDeprecationWarning: The configuration option "asyncio_default_fixture_loop_scope" is unset.
The event loop scope for asynchronous fixtures will default to the fixture caching scope. Future versions of pytest-asyncio will default the loop scope for asynchronous fixtures to function scope. Set the default fixture loop scope explicitly in order to avoid unexpected behavior in the future. Valid fixture loop scopes are: "function", "class", "module", "package", "session"

  warnings.warn(PytestDeprecationWarning(_DEFAULT_FIXTURE_LOOP_SCOPE_UNSET))
=================================================================== test session starts ===================================================================
platform linux -- Python 3.12.7, pytest-8.3.4, pluggy-1.5.0 -- /mnt/datadisk1/freiss/llama/env/bin/python3.12
cachedir: .pytest_cache
rootdir: /mnt/datadisk1/freiss/llama/llama-stack
configfile: pyproject.toml
plugins: asyncio-0.25.0, anyio-4.6.2.post1
asyncio: mode=Mode.STRICT, asyncio_default_fixture_loop_scope=None
collected 9 items                                                                                                                                         

llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_model_list[-vllm] PASSED                                          [ 11%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_completion[-vllm] SKIPPED (Other inference providers don't
support completion() yet)                                                                                                                           [ 22%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_completion_logprobs[-vllm] SKIPPED (Other inference providers
don't support completion() yet)                                                                                                                     [ 33%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_completion_structured_output[-vllm] SKIPPED (This test is not
quite robust)                                                                                                                                       [ 44%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_chat_completion_non_streaming[-vllm] PASSED                       [ 55%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_structured_output[-vllm] SKIPPED (Other inference providers don't
support structured output yet)                                                                                                                      [ 66%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_chat_completion_streaming[-vllm] PASSED                           [ 77%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_chat_completion_with_tool_calling[-vllm] PASSED                   [ 88%]
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_chat_completion_with_tool_calling_streaming[-vllm] PASSED         [100%]

======================================================== 5 passed, 4 skipped, 2 warnings in 25.56s ========================================================
Task was destroyed but it is pending!
task: <Task pending name='Task-6' coro=<AsyncLLMEngine.run_engine_loop() running at /mnt/datadisk1/freiss/llama/env/lib/python3.12/site-packages/vllm/engine/async_llm_engine.py:848> cb=[_log_task_completion(error_callback=<bound method...7cfc479440b0>>)() at /mnt/datadisk1/freiss/llama/env/lib/python3.12/site-packages/vllm/engine/async_llm_engine.py:45, shield.<locals>._inner_done_callback() at /mnt/datadisk1/freiss/llama/env/lib/python3.12/asyncio/tasks.py:905]>
[rank0]:[W1219 11:38:34.689424319 ProcessGroupNCCL.cpp:1250] Warning: WARNING: process group has NOT been destroyed before we destruct ProcessGroupNCCL. On normal program exit, the application should call destroy_process_group to ensure that any pending NCCL operations have finished in this process. In rare cases this process can exit before this point and block the progress of another member of the process group. This constraint has always been present,  but this warning has only been added since PyTorch 2.4 (function operator())

The warning about "asyncio_default_fixture_loop_scope" appears to be due to my environment having a newer version of pytest-asyncio.

The warning about a pending task appears to be due to a bug in vllm.AsyncLLMEngine.shutdown_background_loop(). It looks like that method returns without stopping a pending task. I will look into that issue separately.

Sources

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.