[gemini adapter] 400 INVALID_ARGUMENT when conversation history contains duplicate tool_call_ids

## Observed symptom fix(adapter/gemini): 400 INVALID_ARGUMENT when conversation history contains duplicate tool_call_ids Summary When conversation history forwarded through `/v1/chat/completions` contains duplicate `tool_call_id`s across different turns, routing to Google Gemini / Cloud Code Assist (`antigravity` / `gemini-cli`) causes upstream to return `400 INVALID_ARGUMENT`: ```json { "error": { "code": 400, "message": "Request contains an invalid argument.", "status": "INVALID_ARGUMENT" } } Root Cause In long-running agent workflows (e.g. DeepSeek Harness, Claude Code, Cline), clients may generate or replay identical tool_call_ids in separate turns (for instance, call_51859 generated at Turn 14 and again at Turn 22). The OpenAI-to-Antigravity/Gemini converter serializes these into Gemini contents[].parts[].functionCall and functionResponse elements preserving the raw id. Upstream Google Gemini validates uniqueness of function call identifiers throughout the message history. When duplicate IDs exist, i ## Literal error text ``` 400 INVALID_ARGUMENT duplicate tool_call_ids ``` ## Environment GitHub issue decolua/9router #4532; state=open ## Trigger / context Public GitHub report in decolua/9router (FRESH_7D). ## Source evidence - https://github.com/decolua/9router/issues/4532 (2026-10-02T04:15:44Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #4532 (open) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:40:08.726088Z - scout_id: Ca8d478c550 ## 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.