feat: 添加账号时支持同步上游模型

新增 POST /admin/accounts/models/sync-upstream-preview 端点，
允许在创建账号流程中直接使用凭证同步上游支持的模型，
无需先保存账号获取 ID。

- 后端: 新增 SyncUpstreamModelsPreview handler
- 前端: 新增 syncUpstreamModelsPreview API 函数
- 前端: ModelWhitelistSelector 支持 syncCredentials prop
- 前端: CreateAccountModal 传递凭证给 ModelWhitelistSelector

frontend/src/api/admin/accounts.ts

@@ -487,6 +487,23 @@ export async function syncUpstreamModels(id: number): Promise<SyncUpstreamModels
   return data
 }
 
+export interface SyncUpstreamPreviewParams {
+  platform: string
+  type: string
+  base_url?: string
+  api_key: string
+}
+
+/**
+ * Preview upstream models without a saved account (create-flow)
+ * @param params - Connection credentials
+ * @returns List of model IDs returned by the upstream
+ */
+export async function syncUpstreamModelsPreview(params: SyncUpstreamPreviewParams): Promise<SyncUpstreamModelsResult> {
+  const { data } = await apiClient.post<SyncUpstreamModelsResult>('/admin/accounts/models/sync-upstream-preview', params)
+  return data
+}
+
 export interface CRSPreviewAccount {
   crs_account_id: string
   kind: string
@@ -703,6 +720,7 @@ export const accountsAPI = {
   setSchedulable,
   getAvailableModels,
   syncUpstreamModels,
+  syncUpstreamModelsPreview,
   generateAuthUrl,
   exchangeCode,
   refreshOpenAIToken,