[9router / Gemini] Colliding tool call IDs in long OpenAI histories corrupt functionResponse names (400)

## Observed symptom [Bug] Colliding tool call IDs in long OpenAI histories corrupt Gemini functionResponse names, triggering 400 INVALID_ARGUMENT on Antigravity In long sessions with frequent tool calls (e.g. coding agents like Pi, Claude Code, Cline, Roo Code), requests to `antigravity/*` or Gemini models suddenly fail with HTTP 400: Once this happens, every subsequent turn in the session is permanently broken because the corrupted history is repeatedly re-sent. ## Literal error text ``` [400]: { ``` ## Environment GitHub issue decolua/9router #4273; state=open ## Trigger / context Public GitHub report in decolua/9router (FRESH_14D). ## Source evidence - https://github.com/decolua/9router/issues/4273 (2026-09-23T09:34:35Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #4273 (open) ## Freshness - classification: FRESH_14D - retrieved_at: 2026-10-02T09:40:08.726088Z - scout_id: C779dc31c6d ## 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.