Root cause confirmed via vLLM docs, protocol.py source, RFC #19097, and
GitHub test suite: guided_json was removed in v0.12.0. Our fallback to it
after structured_outputs fails is dead code.
Fix strategy: replace _complete_structured_vllm() with two-tier approach
(response_format as Tier 1, structured_outputs as Tier 2), removing the
dead guided_json path and the chat_template_kwargs merge that may conflict.
Evidence from: vllm.ai docs, vllm-project/vllm tests/entrypoints, protocol.py
to_sampling_params(), PRs #7654#9530#15627, RFC #19097
Woody