fix: 修复 fetch.ts preconnect 重复标识符类型错误

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/costrict/provider/fetch.ts

@@ -43,8 +43,13 @@ const VERSION = getVersion()
  * 3. 注入 Authorization 和 CoStrict 特有 headers
  * 4. 反应性 401 错误恢复（自动重试一次）
  */
+type CoStrictFetchFn = {
+  (input: RequestInfo | URL, init?: RequestInit): Promise<Response>
+  preconnect?: (url: string | URL) => void
+}
+
 export function createCoStrictFetch() {
-  const costrictFetch = async (
+  const costrictFetch: CoStrictFetchFn = async (
     input: RequestInfo | URL,
     init?: RequestInit,
   ) => {