fix: Get builtin tool calling working in remote-vllm

[bbrowning]

What does this PR do?

This PR makes a couple of changes required to get the test tests/client-sdk/agents/test_agents.py::test_builtin_tool_web_search passing on the remote-vllm provider.

First, we adjust agent_instance to also pass in the description and parameters of builtin tools. We need these parameters so we can pass the tool's expected parameters into vLLM. The meta-reference implementations may not have needed these for builtin tools, as they are able to take advantage of the Llama-model specific support for certain builtin tools. However, with vLLM, our server-side chat templates for tool calling treat all tools the same and don't separate out Llama builtin vs custom tools. So, we need to pass the full set of parameter definitions and list of required parameters for builtin tools as well.

Next, we adjust the vllm streaming chat completion code to fix up some edge cases where it was returning an extra ChatCompletionResponseEvent with an empty ToolCall with empty string call_id, tool_name, and arguments properties. This is a bug discovered after the above fix, where after a successful tool invocation we were sending extra chunks back to the client with these empty ToolCalls.

Test Plan

With these changes, the following test that previously failed now passes:

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/client-sdk/agents/test_agents.py::test_builtin_tool_web_search \
--inference-model "meta-llama/Llama-3.2-3B-Instruct"

Additionally, I ran the remote-vllm client-sdk and provider inference tests as below to ensure they all still passed with this change:

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/client-sdk/inference/test_text_inference.py \
--inference-model "meta-llama/Llama-3.2-3B-Instruct"

VLLM_URL="http://localhost:8000/v1" \
python -m pytest -s -v \
llama_stack/providers/tests/inference/test_text_inference.py \
--providers "inference=vllm_remote"

[terrytangyuan]

@ashwinb @yanxi0830 Could you help double check on this change?

[ehhuang]

I'm not sure if having additional fields here would break things (seems not?), but if not, looks like we can just set the key value here for tool_def_map then L926 below should take care fo this.