fix: Qwen3 XML streaming tool-call boundary fix

[jkyamog]

Purpose

Fix phantom empty tool calls and malformed arguments in streamed Qwen3 XML
tool calls. When a parser chunk contained a closing tag followed by a new call
opening, the fallback closing logic could act on the newly opened call in the
same chunk. That produced the observed {}/empty-name calls and extra closing
braces in tool arguments.

The change captures the call ID at chunk entry and restricts fallback closure
to that call; the function fallback also requires an open function. It keeps the
native Qwen3 XML dialect and templates unchanged.

Test Plan

Run from the repository root in the parser/test container:

pytest -q tests/tool_parsers/test_qwen3xml_chunk_boundaries.py \
  tests/tool_parsers/test_qwen3xml_tool_parser.py
ruff check vllm/tool_parsers/qwen3xml_tool_parser.py \
  tests/tool_parsers/test_qwen3xml_chunk_boundaries.py
git diff --check origin/main...HEAD

Runtime contract checks:

python vllm/tmp/probe_stream_contract.py
node vllm/tmp/probe_pi_stream.mjs qwen-3.8-flash

Verify streamed and non-streamed responses produce valid JSON tool arguments,
no empty-name calls, preserved reasoning when enabled, and the exact requested
tool sequence read(path=/tmp/a) followed by bash(command=pwd).

Test Result

• The expanded parser suite passed all 535 cases, including fixed-width,
  single-split, seeded-irregular chunk boundaries, public
  streaming/non-streaming agreement, empty arguments, and multiline values.
• Ruff checks and changed-file format checks passed.
• Repeated live Ark/Qwen streams produced exactly the requested read and
  bash calls with valid JSON, preserved reasoning, and
  finish_reason=tool_calls.
• The Pi 0.84.4 adapter returned stopReason=toolUse with the same two calls;
  no generated tool request was executed.
• The final combined-image NVIDIA Flash-Next stream reproduced the same
  parser contract, showing that the fix is independent of the Ark runtime.

The latest-upstream stack has been rebuilt locally alongside the NVIDIA loader;
rerun the focused suite and final live smoke after selecting that image before
submitting the PR.

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[areslp]

Independent validation: this fixes a real OpenClaw tool-calling failure on our 1Cat-vLLM 1.5.0 deployment. Thank you for the patch; I would welcome its inclusion in a release.

We tested the parser change at ae22169a953064cdec8befb5a09630674247ed67 without additional parser modifications, using Qwen3.8-Flash-Next-NVFP4 on 4× V100, TP4, MTP4, and Python 3.12.13.

Before the fix: the model generated two complete XML tool calls, but the streamed response contained three slots: a valid call, an empty unnamed call, and a call with an extra } in its JSON arguments. OpenClaw rejected it with Provider returned an incomplete or malformed tool call. The response had a complete [DONE] terminator and no transport error.

Independent results:

• Replaying the captured tool-call text at fixed chunk widths 1–200: the installed parser failed semantic validation at 113/200 widths, with phantom slots at 29 widths. With this patch: 200/200 passed, no phantom slots. The outer streaming parser also passed 200/200 widths.
• Nine additional chunk strategies exercised through both the inner and outer parsers: 18/18 passed with the patch.
• Our separate synthetic adjacent-call regression tests: 201 failed / 200 passed before the fix. With this PR's boundary tests plus ours: 936 passed, using pytest 8.4.2. Ruff checks and git diff --check also passed.
• After loading the exact patched parser via --tool-parser-plugin, two streaming requests and one non-streaming request through OpenClaw's installed model transport each returned exactly the two requested synthetic tool calls, valid JSON, correct typed argument values, and finish_reason=tool_calls. No generated tool was executed.

The replay uses actual captured text with deterministic synthetic chunk boundaries, not a reconstruction of the original server chunk timing. One segmentation reproduced the exact malformed argument hashes seen on the wire. The live checks are focused protocol smoke tests, not a long-term reliability claim. Private conversation contents are not included here.

The repair is now deployed in our environment. This investigation and validation report were AI-assisted. These results support the value of fixing the legacy parser still shipped in 1Cat 1.5.0, even though upstream vLLM has moved to a replacement parser engine.