fix: Gracefully handle no choices in remote vLLM response

[bbrowning]

What does this PR do?

This gracefully handles the case where the vLLM server responded to a completion request with no choices, which can happen in certain vLLM error situations. Previously, we'd error out with a stack trace about a list index out of range. Now, we just log a warning to the user and move past any chunks with an empty choices list.

A specific example of the type of stack trace this fixes:

  File "/app/llama-stack-source/llama_stack/providers/remote/inference/vllm/vllm.py", line 170, in _process_vllm_chat_completion_stream_response
    choice = chunk.choices[0]
             ~~~~~~~~~~~~~^^^
IndexError: list index out of range

Now, instead of erroring out with that stack trace, we log a warning that vLLM failed to generate any completions and alert the user to check the vLLM server logs for details.

This is related to #1277 and addresses the stack trace shown in that issue, although does not in and of itself change the functional behavior of vLLM tool calling.

Test Plan

As part of this fix, I added new unit tests to trigger this same error and verify it no longer happens. That is
test_process_vllm_chat_completion_stream_response_no_choices in the new tests/unit/providers/inference/test_remote_vllm.py. I also added a couple of more tests to trigger and verify the last couple of remote vllm provider bug fixes - specifically a test for #1236 (builtin tool calling) and #1325 (vLLM <= v0.6.3).

This required fixing the signature of
_process_vllm_chat_completion_stream_response to accept the actual type of chunks it was getting passed - specifically changing from our openai_compat OpenAICompatCompletionResponse to openai.types.chat.chat_completion_chunk.ChatCompletionChunk. It was not actually getting passed OpenAICompatCompletionResponse objects before, and was using attributes that didn't exist on those objects. So, the signature now matches the type of object it's actually passed.

Run these new unit tests like this:

pytest tests/unit/providers/inference/test_remote_vllm.py

Additionally, I ensured the existing test_text_inference.py tests passed via:

VLLM_URL="http://localhost:8000/v1" \
INFERENCE_MODEL="meta-llama/Llama-3.2-3B-Instruct" \
LLAMA_STACK_CONFIG=remote-vllm \
python -m pytest -v tests/integration/inference/test_text_inference.py \
--inference-model "meta-llama/Llama-3.2-3B-Instruct" \
--vision-inference-model ""

[terrytangyuan]

Thank you!

[leseb]

s/generation/generate/?