fix(gemini): 更新 Gemini 模型列表配置

- 移除已弃用的 1.5 系列模型
- 调整模型优先级顺序（2.0 Flash > 2.5 Flash > 2.5 Pro > 3.0 Preview）
- 同步前后端模型配置
- 更新相关测试用例和默认模型选择逻辑

frontend/src/components/admin/account/AccountTestModal.vue

@@ -232,8 +232,11 @@ const loadAvailableModels = async () => {
     if (availableModels.value.length > 0) {
       if (props.account.platform === 'gemini') {
         const preferred =
+          availableModels.value.find((m) => m.id === 'gemini-2.0-flash') ||
+          availableModels.value.find((m) => m.id === 'gemini-2.5-flash') ||
           availableModels.value.find((m) => m.id === 'gemini-2.5-pro') ||
-          availableModels.value.find((m) => m.id === 'gemini-3-pro')
+          availableModels.value.find((m) => m.id === 'gemini-3-flash-preview') ||
+          availableModels.value.find((m) => m.id === 'gemini-3-pro-preview')
         selectedModelId.value = preferred?.id || availableModels.value[0].id
       } else {
         // Try to select Sonnet as default, otherwise use first model