fix: 默认补全 antigravity 的 Gemini 3.1 Pro 透传映射

backend/internal/service/account.go

@@ -373,7 +373,11 @@ func (a *Account) GetModelMapping() map[string]string {
 		}
 		if len(result) > 0 {
 			if a.Platform == domain.PlatformAntigravity {
-				ensureAntigravityDefaultPassthrough(result, "gemini-3-flash")
+				ensureAntigravityDefaultPassthroughs(result, []string{
+					"gemini-3-flash",
+					"gemini-3.1-pro-high",
+					"gemini-3.1-pro-low",
+				})
 			}
 			return result
 		}
@@ -400,6 +404,12 @@ func ensureAntigravityDefaultPassthrough(mapping map[string]string, model string
 	mapping[model] = model
 }
 
+func ensureAntigravityDefaultPassthroughs(mapping map[string]string, models []string) {
+	for _, model := range models {
+		ensureAntigravityDefaultPassthrough(mapping, model)
+	}
+}
+
 // IsModelSupported 检查模型是否在 model_mapping 中（支持通配符）
 // 如果未配置 mapping，返回 true（允许所有模型）
 func (a *Account) IsModelSupported(requestedModel string) bool {