なぜ Gemini 画像で IMAGE_OTHER が返されるのか？

{
  "candidates": [
    {
      "finishReason": "IMAGE_OTHER",
      "finishMessage": "Unable to show the generated image. The model could not generate the image based on the prompt provided. You will not be charged for this request. Try rephrasing the prompt. ..."
    }
  ],
  "usageMetadata": {
    "promptTokenCount": 1430,
    "candidatesTokenCount": 274,
    "thoughtsTokenCount": 274
  },
  "modelVersion": "gemini-3-pro-image",
  "responseId": "..."
}

def classify_no_image(resp: dict) -> str:
    if resp.get("promptFeedback", {}).get("blockReason"):
        return "input_blocked"        # see blockReason: OTHER troubleshooting
    cand = (resp.get("candidates") or [{}])[0]
    reason = cand.get("finishReason")
    if reason == "IMAGE_OTHER":
        return "output_filtered"      # rewrite names / restricted descriptions
    if reason == "NO_IMAGE":
        return "no_image_intent"      # see NO_IMAGE troubleshooting
    if reason in ("IMAGE_SAFETY", "IMAGE_PROHIBITED_CONTENT"):
        return "safety"               # explicit safety block, do not retry
    return "ok" if any("inlineData" in p for p in (cand.get("content") or {}).get("parts") or []) else "unknown"