fix: Fixed an "out of token budget" error when attempting a tool call via remote vLLM provider (#2114)

# What does this PR do?
Closes #2113.
Closes #1783.

Fixes a bug in handling the end of tool execution request stream where
no `finish_reason` is provided by the model.

## Test Plan
1. Ran existing unit tests
2. Added a dedicated test verifying correct behavior in this edge case
3. Ran the code snapshot from #2113

[//]: # (## Documentation)

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

@@ -158,33 +158,29 @@ def _convert_to_vllm_finish_reason(finish_reason: str) -> StopReason:
     }.get(finish_reason, StopReason.end_of_turn)
 
 
-async def _process_vllm_chat_completion_stream_response(
-    stream: AsyncGenerator[OpenAIChatCompletionChunk, None],
-) -> AsyncGenerator:
-    event_type = ChatCompletionResponseEventType.start
-    tool_call_buf = UnparseableToolCall()
-    async for chunk in stream:
-        if not chunk.choices:
-            log.warning("vLLM failed to generation any completions - check the vLLM server logs for an error.")
-            continue
-        choice = chunk.choices[0]
-        if choice.delta.tool_calls:
-            tool_call = convert_tool_call(choice.delta.tool_calls[0])
-            tool_call_buf.tool_name += str(tool_call.tool_name)
-            tool_call_buf.call_id += tool_call.call_id
-            # TODO: remove str() when dict type for 'arguments' is no longer allowed
-            tool_call_buf.arguments += str(tool_call.arguments)
-        if choice.finish_reason:
-            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:
-                yield ChatCompletionResponseStreamChunk(
+def _process_vllm_chat_completion_end_of_stream(
+    finish_reason: str | None,
+    last_chunk_content: str | None,
+    current_event_type: ChatCompletionResponseEventType,
+    tool_call_buf: UnparseableToolCall,
+) -> list[OpenAIChatCompletionChunk]:
+    chunks = []
+
+    if finish_reason is not None:
+        stop_reason = _convert_to_vllm_finish_reason(finish_reason)
+    else:
+        stop_reason = StopReason.end_of_message
+
+    if tool_call_buf.tool_name:
+        # at least one tool call request is received
+
+        args_str = tool_call_buf.arguments or "{}"
+        try:
+            args = json.loads(args_str)
+            chunks.append(
+                ChatCompletionResponseStreamChunk(
                     event=ChatCompletionResponseEvent(
-                        event_type=event_type,
+                        event_type=current_event_type,
                         delta=ToolCallDelta(
                             tool_call=ToolCall(
                                 call_id=tool_call_buf.call_id,
@@ -196,8 +192,12 @@ async def _process_vllm_chat_completion_stream_response(
                         ),
                     )
                 )
-            elif args_str:
-                yield ChatCompletionResponseStreamChunk(
+            )
+        except Exception as e:
+            log.warning(f"Failed to parse tool call buffer arguments: {args_str} \nError: {e}")
+
+            chunks.append(
+                ChatCompletionResponseStreamChunk(
                     event=ChatCompletionResponseEvent(
                         event_type=ChatCompletionResponseEventType.progress,
                         delta=ToolCallDelta(
@@ -206,14 +206,50 @@ async def _process_vllm_chat_completion_stream_response(
                         ),
                     )
                 )
-            yield ChatCompletionResponseStreamChunk(
-                event=ChatCompletionResponseEvent(
-                    event_type=ChatCompletionResponseEventType.complete,
-                    delta=TextDelta(text=choice.delta.content or ""),
-                    logprobs=None,
-                    stop_reason=_convert_to_vllm_finish_reason(choice.finish_reason),
-                )
             )
+
+    chunks.append(
+        ChatCompletionResponseStreamChunk(
+            event=ChatCompletionResponseEvent(
+                event_type=ChatCompletionResponseEventType.complete,
+                delta=TextDelta(text=last_chunk_content or ""),
+                logprobs=None,
+                stop_reason=stop_reason,
+            )
+        )
+    )
+
+    return chunks
+
+
+async def _process_vllm_chat_completion_stream_response(
+    stream: AsyncGenerator[OpenAIChatCompletionChunk, None],
+) -> AsyncGenerator:
+    event_type = ChatCompletionResponseEventType.start
+    tool_call_buf = UnparseableToolCall()
+    end_of_stream_processed = False
+
+    async for chunk in stream:
+        if not chunk.choices:
+            log.warning("vLLM failed to generation any completions - check the vLLM server logs for an error.")
+            return
+        choice = chunk.choices[0]
+        if choice.delta.tool_calls:
+            tool_call = convert_tool_call(choice.delta.tool_calls[0])
+            tool_call_buf.tool_name += str(tool_call.tool_name)
+            tool_call_buf.call_id += tool_call.call_id
+            # TODO: remove str() when dict type for 'arguments' is no longer allowed
+            tool_call_buf.arguments += str(tool_call.arguments)
+        if choice.finish_reason:
+            chunks = _process_vllm_chat_completion_end_of_stream(
+                finish_reason=choice.finish_reason,
+                last_chunk_content=choice.delta.content,
+                current_event_type=event_type,
+                tool_call_buf=tool_call_buf,
+            )
+            for c in chunks:
+                yield c
+            end_of_stream_processed = True
         elif not choice.delta.tool_calls:
             yield ChatCompletionResponseStreamChunk(
                 event=ChatCompletionResponseEvent(
@@ -224,6 +260,17 @@ async def _process_vllm_chat_completion_stream_response(
             )
             event_type = ChatCompletionResponseEventType.progress
 
+    if end_of_stream_processed:
+        return
+
+    # the stream ended without a chunk containing finish_reason - we have to generate the
+    # respective completion chunks manually
+    chunks = _process_vllm_chat_completion_end_of_stream(
+        finish_reason=None, last_chunk_content=None, current_event_type=event_type, tool_call_buf=tool_call_buf
+    )
+    for c in chunks:
+        yield c
+
 
 class VLLMInferenceAdapter(Inference, ModelsProtocolPrivate):
     def __init__(self, config: VLLMInferenceAdapterConfig) -> None: