[openai-agents] Cannot run Databricks-hosted Gemini models (wrong gateway surface / dropped thoughts)

## Observed symptom [Bug] openai-agents can't run Databricks-hosted Gemini models (wrong gateway surface, dropped thought signatures, looping tool calls) Description Pointing an `openai-agents` agent at a Databricks-hosted Gemini model (`databricks-gemini-3-8-flash` / `system.ai.gemini-3-8-flash`) with `auth: {type: databricks, profile: …}` fails in three stacked ways: 1. **Wrong gateway surface.** The harness reuses ucode's codex base URL (`/ai-gateway/codex/v1`). That surface only proxies GPT's Responses API: chat completions there return 400 ("doesn't match any known API type"), and Responses returns "Responses API passthrough is not supported for model databricks-gemini-…". The model-services API lists Gemini on `mlflow/v1/chat/completions`, and `/ai-gateway/mlflow/v1/chat/completions` works. 2. **Thought signatures are dropped.** Gemini 3 rejects the second step of any tool call with `Function call is missing a thought_signature in functionCall parts`. The gateway returns the signature as `tool_calls[].thoughtSignature` (also in stre ## Version info Version: 0.17 ## Environment GitHub issue omnigent-ai/omnigent #8654; state=OPEN ## Trigger / context Public GitHub report in omnigent-ai/omnigent (FRESH_7D). ## Source evidence - https://github.com/omnigent-ai/omnigent/issues/8654 (2026-10-01T00:08:36Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #8654 (OPEN) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:33:42Z - scout_id: Cb605890058 ## 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.