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/composables/useModelWhitelist.ts

@@ -43,13 +43,13 @@ export const claudeModels = [
 
 // Google Gemini
 const geminiModels = [
-  'gemini-2.0-flash', 'gemini-2.0-flash-lite-preview', 'gemini-2.0-flash-exp',
-  'gemini-2.0-pro-exp', 'gemini-2.0-flash-thinking-exp',
-  'gemini-2.5-pro-exp-03-25', 'gemini-2.5-pro-preview-03-25',
-  'gemini-3-pro-preview',
-  'gemini-1.5-pro', 'gemini-1.5-pro-latest',
-  'gemini-1.5-flash', 'gemini-1.5-flash-latest', 'gemini-1.5-flash-8b',
-  'gemini-exp-1206'
+  // Keep in sync with backend curated Gemini lists.
+  // This list is intentionally conservative (models commonly available across OAuth/API key).
+  'gemini-2.0-flash',
+  'gemini-2.5-flash',
+  'gemini-2.5-pro',
+  'gemini-3-flash-preview',
+  'gemini-3-pro-preview'
 ]
 
 // 智谱 GLM
@@ -229,9 +229,8 @@ const openaiPresetMappings = [
 
 const geminiPresetMappings = [
   { label: 'Flash 2.0', from: 'gemini-2.0-flash', to: 'gemini-2.0-flash', color: 'bg-blue-100 text-blue-700 hover:bg-blue-200 dark:bg-blue-900/30 dark:text-blue-400' },
-  { label: 'Flash Lite', from: 'gemini-2.0-flash-lite-preview', to: 'gemini-2.0-flash-lite-preview', color: 'bg-indigo-100 text-indigo-700 hover:bg-indigo-200 dark:bg-indigo-900/30 dark:text-indigo-400' },
-  { label: '1.5 Pro', from: 'gemini-1.5-pro', to: 'gemini-1.5-pro', color: 'bg-purple-100 text-purple-700 hover:bg-purple-200 dark:bg-purple-900/30 dark:text-purple-400' },
-  { label: '1.5 Flash', from: 'gemini-1.5-flash', to: 'gemini-1.5-flash', color: 'bg-emerald-100 text-emerald-700 hover:bg-emerald-200 dark:bg-emerald-900/30 dark:text-emerald-400' }
+  { label: '2.5 Flash', from: 'gemini-2.5-flash', to: 'gemini-2.5-flash', color: 'bg-indigo-100 text-indigo-700 hover:bg-indigo-200 dark:bg-indigo-900/30 dark:text-indigo-400' },
+  { label: '2.5 Pro', from: 'gemini-2.5-pro', to: 'gemini-2.5-pro', color: 'bg-purple-100 text-purple-700 hover:bg-purple-200 dark:bg-purple-900/30 dark:text-purple-400' }
 ]
 
 // =====================