fix(acceptance): 优先选择稳定版 Gemini 图片模型

生产同构验收默认优先选择当前稳定版 gemini-3.1-flash-image，并在回退选择中将非 preview 模型置于预览版之前，避免选中并发 5 的预览候选后与千请求八分钟门槛产生物理冲突。\n\n候选确定后的正式路由优先级与真实限流策略保持不变。\n\n验证：bash -n；ShellCheck；git diff --check。

scripts/cluster/run-production-acceptance.sh

@@ -371,7 +371,13 @@ WHERE platform.status='enabled' AND platform.deleted_at IS NULL
   AND lower(platform.provider) IN ('gemini','google-gemini','gemini-openai')
   AND model.model_type @> '[\"image_edit\"]'::jsonb
 ORDER BY
-  CASE WHEN lower(b.invocation_name) LIKE '%gemini%image%' THEN 0 ELSE 1 END,
+  CASE
+    WHEN lower(b.invocation_name) = 'gemini-3.1-flash-image' THEN 0
+    WHEN lower(b.invocation_name) LIKE '%gemini%image%'
+      AND lower(b.invocation_name) NOT LIKE '%preview%' THEN 1
+    WHEN lower(b.invocation_name) LIKE '%gemini%image%' THEN 2
+    ELSE 3
+  END,
   platform.priority, model.created_at
 LIMIT 1;")
   fi