DEV: Add a test to ensure that our SMTP settings are correct (#26410)

Why this change?

This is a follow up to 897be75941eb95b89e885e4b447ddf0c75eb81ff.

When updating `net-smtp` from `0.4.x` to `0.5.x`, our test suite passed
but the error `ArgumentError: SMTP-AUTH requested but missing user name`
was being thrown in production leading to emails being failed to send
out via SMTP.

This commit adds a test to ensure that our production SMTP settings will
at least attemp to connect to an SMTP server.

spec/integration/smtp_spec.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+RSpec.describe "SMTP Settings Integration" do
+  before do
+    @original_action_mailer_smtp_settings = ActionMailer::Base.smtp_settings
+    @original_action_mailer_delivery_method = ActionMailer::Base.delivery_method
+    ActionMailer::Base.delivery_method = :smtp
+  end
+
+  after do
+    ActionMailer::Base.smtp_settings = @original_action_mailer_smtp_settings
+    ActionMailer::Base.delivery_method = @original_action_mailer_delivery_method
+  end
+
+  it "should send out the email successfully using the SMTP settings" do
+    global_setting :smtp_address, "some.host"
+    global_setting :smtp_port, 12_345
+
+    ActionMailer::Base.smtp_settings = GlobalSetting.smtp_settings
+
+    message = TestMailer.send_test("some_email")
+
+    expect do Email::Sender.new(message, :test_message).send end.to raise_error(
+      StandardError,
+      /some.host/,
+    )
+  end
+end