feat(invoice): 添加发票发送功能并进行测试

- 在 InvoiceRepo 中添加 send 方法，用于发送发票邮件
- 在 CreateInvoiceTest 中添加测试用例，验证发票发送功能

tests/Feature/Invoice/CreateInvoiceTest.php

@@ -34,3 +34,14 @@ it('should can create invoice', function () {
     expect($invoice)
         ->toBeInstanceOf(Invoice::class);
 });
+
+it('should can send invoice email to receiver', function () {
+    $repo = make(InvoiceRepo::class);
+
+    expect((function () use ($repo) {
+        $invoice_no = '517268';
+        $repo->send($invoice_no);
+
+        return true;
+    })())->not->toThrow(Throwable::class);
+});