fix: More robust handling of the arguments in tool call response in remote::vllm (#1169)

# What does this PR do?

This fixes the following issue on the server side when the tool call
response contains empty args. This happens when running
`examples.agents.e2e_loop_with_client_tools` but `get_ticker_data`
returns `[]`:

```
Traceback (most recent call last):
  File "/home/yutang/repos/llama-stack/llama_stack/distribution/server/server.py", line 208, in sse_generator
    async for item in event_gen:
  File "/home/yutang/repos/llama-stack/llama_stack/providers/inline/agents/meta_reference/agents.py", line 169, in _create_agent_turn_streaming
    async for event in agent.create_and_execute_turn(request):
  File "/home/yutang/repos/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 189, in create_and_execute_turn
    async for chunk in self.run(
  File "/home/yutang/repos/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 258, in run
    async for res in self._run(
  File "/home/yutang/repos/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 499, in _run
    async for chunk in await self.inference_api.chat_completion(
  File "/home/yutang/repos/llama-stack/llama_stack/distribution/routers/routers.py", line 182, in <genexpr>
    return (chunk async for chunk in await provider.chat_completion(**params))
  File "/home/yutang/repos/llama-stack/llama_stack/providers/remote/inference/vllm/vllm.py", line 296, in _stream_chat_completion
    async for chunk in res:
  File "/home/yutang/repos/llama-stack/llama_stack/providers/remote/inference/vllm/vllm.py", line 162, in _process_vllm_chat_completion_stream_response
    arguments=json.loads(tool_call_buf.arguments),
  File "/home/yutang/.conda/envs/distribution-myenv/lib/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/home/yutang/.conda/envs/distribution-myenv/lib/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/home/yutang/.conda/envs/distribution-myenv/lib/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```
## Test Plan

All existing tests in
`tests/client-sdk/inference/test_text_inference.py` passed.

[//]: # (## Documentation)

---------

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

llama_stack/providers/remote/inference/vllm/vllm.py

@@ -148,19 +148,36 @@ async def _process_vllm_chat_completion_stream_response(
     async for chunk in stream:
         choice = chunk.choices[0]
         if choice.finish_reason:
-            yield ChatCompletionResponseStreamChunk(
-                event=ChatCompletionResponseEvent(
-                    event_type=event_type,
-                    delta=ToolCallDelta(
-                        tool_call=ToolCall(
-                            call_id=tool_call_buf.call_id,
-                            tool_name=tool_call_buf.tool_name,
-                            arguments=json.loads(tool_call_buf.arguments),
+            args_str = tool_call_buf.arguments
+            args = None
+            try:
+                args = {} if not args_str else json.loads(args_str)
+            except Exception as e:
+                log.warning(f"Failed to parse tool call buffer arguments: {args_str} \nError: {e}")
+            if args is not None:
+                yield ChatCompletionResponseStreamChunk(
+                    event=ChatCompletionResponseEvent(
+                        event_type=event_type,
+                        delta=ToolCallDelta(
+                            tool_call=ToolCall(
+                                call_id=tool_call_buf.call_id,
+                                tool_name=tool_call_buf.tool_name,
+                                arguments=args,
+                            ),
+                            parse_status=ToolCallParseStatus.succeeded,
                         ),
-                        parse_status=ToolCallParseStatus.succeeded,
-                    ),
+                    )
+                )
+            else:
+                yield ChatCompletionResponseStreamChunk(
+                    event=ChatCompletionResponseEvent(
+                        event_type=ChatCompletionResponseEventType.progress,
+                        delta=ToolCallDelta(
+                            tool_call=str(tool_call_buf),
+                            parse_status=ToolCallParseStatus.failed,
+                        ),
+                    )
                 )
-            )
             yield ChatCompletionResponseStreamChunk(
                 event=ChatCompletionResponseEvent(
                     event_type=ChatCompletionResponseEventType.complete,