[sglang / gpt-oss] tool_choice:"required" returns raw harmony control tokens as assistant content

## Observed symptom [Bug] gpt-oss: `tool_choice: "required"` returns raw harmony control tokens as assistant content - [x] I searched related issues but found no solution. - [x] The bug persists in the latest version. - [x] Issues without environment info and a minimal reproducible demo are hard to resolve and may receive no feedback. - [x] If this is not a bug report but a general question, please start a discussion at https://github.com/sgl-project/sglang/discussions. Otherwise, it will be closed. - [x] Please use English. Otherwise, it will be closed. When serving `openai/gpt-oss-20b` with its own `--tool-call-parser gpt-oss`, a **forced** tool choice comes back as HTTP 200 with `finish_reason: "stop"`, no `tool_calls`, and the harmony control tokens handed to the client as assistant `content`: ## Environment GitHub issue sgl-project/sglang #41747 ## Trigger / context Public GitHub report in sgl-project/sglang (FRESH_7D). ## Source evidence - https://github.com/sgl-project/sglang/issues/41747 (2026-09-29T19:45:27Z) — evidence_class: PRIMARY_ISSUE_REPORT — #41747 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: C4fa7c2c56e ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.