サムネ取得 API 見直し (#398) (#402)

Reviewed-on: http://git.miteruzo.com/miteruzo/btrc-hub/pulls/402
Co-authored-by: miteruzo <miteruzo@naver.com>
Co-committed-by: miteruzo <miteruzo@naver.com>

backend/spec/services/preview/url_safety_spec.rb

@@ -0,0 +1,15 @@
+require 'rails_helper'
+
+RSpec.describe Preview::UrlSafety do
+  describe '.validate' do
+    it 'raises UnsafeUrl when DNS resolution fails' do
+      allow(Resolv).to receive(:getaddresses)
+          .with('missing.example')
+          .and_raise(Resolv::ResolvError)
+
+      expect {
+        described_class.validate('https://missing.example')
+      }.to raise_error(Preview::UrlSafety::UnsafeUrl)
+    end
+  end
+end