fix(vllm): non-streaming tool-call regression after #10351 (#10638)

fix(vllm): non-streaming tool-call regression after #10351 (native_streaming is a capability flag, not a state flag)

#10351 introduced native streaming via `parser.extract_tool_calls_streaming`
and gated the post-loop `extract_tool_calls` block on `native_streaming and
not native_streaming_error`. That works for streaming requests, but for
non-streaming requests the same flag is still True (it only means "the
parser can stream", not "we actually streamed"), so the block was skipped
and the `elif` cleared `content = ""` — the tool call was silently lost.

Symptom: non-streaming chat.completions with `tools=[...]` returns
`finish_reason: "stop"` with `content: ""` and no `tool_calls`. Streaming
requests are unaffected.

Fix: gate both branches on `streaming` too, so the extract_tool_calls
block runs for non-streaming requests (and for streaming requests that
fell back to the buffered path).

Reproduction (vLLM 0.24, Qwen3-Coder-Next-NVFP4, qwen3_coder parser):

    curl -s -X POST http://localhost:8080/v1/chat/completions \
      -H 'Content-Type: application/json' \
      -d '{"model":"coder","stream":false,
           "messages":[{"role":"user","content":"7*8 via calc"}],
           "tools":[{"type":"function","function":{"name":"calc",
             "parameters":{"type":"object",
               "properties":{"expression":{"type":"string"}}}}}]}'

Before: finish_reason: "stop", content: "", tool_calls: []
After:  finish_reason: "tool_calls", tool_calls[0].function.name: "calc"

Streaming path re-verified in the same setup: delta.tool_calls arrives
token-by-token, finish_reason: "tool_calls", no raw XML in content.

Signed-off-by: pos-ei-don <1822533+pos-ei-don@users.noreply.github.com>

backend/python/vllm/backend.py

@@ -748,7 +748,12 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
         # When (A) native streaming ran cleanly, per-delta yields above already
         # delivered everything — do NOT extract again on the full text or we'd
         # duplicate content/tool_calls into the final chunk.
-        if has_tool_parser and not (native_streaming and not native_streaming_error):
+        # NOTE: `native_streaming` is a capability flag ("streaming parser is
+        # available"), not a state flag ("streaming actually ran"). For
+        # non-streaming requests it is still True but the per-delta loop was
+        # never entered, so we MUST still run extract_tool_calls here. Hence
+        # the explicit `streaming and …` guard on both branches.
+        if has_tool_parser and not (streaming and native_streaming and not native_streaming_error):
             try:
                 tp = tp_instance
                 if tp is None:
@@ -770,7 +775,7 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
                         ))
             except Exception as e:
                 print(f"Tool parser error: {e}", file=sys.stderr)
-        elif native_streaming and not native_streaming_error:
+        elif streaming and native_streaming and not native_streaming_error:
             # Per-delta path already emitted content + tool_calls; the final
             # chat_delta should carry only metadata (token counts, logprobs).
             content = ""