[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 markdown ### 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. U ## Literal error text ``` 400 INVALID_ARGUMENT duplicate tool_call_ids ``` ## Environment GitHub issue decolua/9router #4532; state=open ## Trigger / context markdown ### 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, it rejects the entire request with INVALID_ARGUMENT. Steps to Reproduce Send an OpenAI-compatible request to an antigravity or gemini model containing repeated tool call IDs across turns: bash curl -X POST http://localhost:20128/v1/chat/completions \ -H "Content-Type: application/json" \ -H "Authorization: Bearer <API_KEY>" \ -d '{ "model": "combo-solid", "messages": [ { "role": "user", "content": "run step 1" }, { "role": "assistant", "tool_calls": [{ "id": "call_51859", "type": "function", "function": { "name": "bash", "arguments": " ## Source evidence - https://github.com/decolua/9router/issues/4532 (2026-10-02T04:15:44Z) — 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.