[Bugfix] Stop xgrammar advancement after grammar termination

[alexbi29]

Summary

Small hardening patch for the xgrammar structured-output backend. When speculative decoding hands multiple accepted tokens to XgrammarGrammar, stop advancing as soon as xgrammar reports that the matcher has terminated. This avoids feeding later speculative tokens, commonly token 198 (\n), into an already terminated matcher.

This is related to upstream vLLM PRs:

• [Bugfix] Sync xgrammar termination state after failed token acceptance vllm-project/vllm#37506: similar area, but it syncs _is_terminated only after xgrammar has already rejected the extra token.
• [Bugfix][Structured Output][Spec Decode] Constrain bitmask and trim grammar advance at the reasoning boundary vllm-project/vllm#44297: broader structured-output/spec-decode reasoning-boundary fix for [Bug]: [structured outputs] speculative decoding + VLLM_ENFORCE_STRICT_TOOL_CALLING=1 failed to advance FSM vllm-project/vllm#44006.

Local live validation

On qwen3.6-27b with MTP/spec decode, strict Qwen3Coder tool calling, and the patched branch loaded:

• 18/18 concurrent default tool-call requests returned finish_reason=tool_calls
• New log scan after the burst: new_xgrammar_warnings 0

Before the patch, the same class of requests produced repeated:

grammar_matcher.cc:612: Warning: The matcher has terminated after accepting the stop token, but is trying to accept new token with id 198.

Tests

/home/ubuntu/vllm-env/bin/python -m pytest \
  tests/v1/structured_output/test_backend_xgrammar.py \
  tests/v1/structured_output/test_utils.py -q

Result: 8 passed, 16 warnings.

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