fix: 修复支付宝签名时错误排除 sign_type 导致 invalid-signature

支付宝验签字符串包含 sign_type 字段，但 generateSign 错误地
将 sign_type 与 sign 一起排除。只需排除 sign，保留 sign_type。

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

src/lib/alipay/client.ts

@@ -51,7 +51,7 @@ export function pageExecute(
 
   params.sign = generateSign(params, env.ALIPAY_PRIVATE_KEY);
 
-  const query = new URLSearchParams({ ...params, sign_type: 'RSA2' }).toString();
+  const query = new URLSearchParams(params).toString();
   return `${GATEWAY}?${query}`;
 }
 
@@ -72,7 +72,6 @@ export async function execute<T extends AlipayResponse>(
   };
 
   params.sign = generateSign(params, env.ALIPAY_PRIVATE_KEY);
-  params.sign_type = 'RSA2';
 
   const response = await fetch(GATEWAY, {
     method: 'POST',