test: add RSS feed endpoint tests

Verify content type, RSS structure, draft exclusion, and language
element.

test/server_test.rb

@@ -92,4 +92,15 @@ class ServerTest < Minitest::Test
     assert_includes slugs, "global"
     refute_includes slugs, "hello"
   end
+
+  def test_feed_endpoint
+    get "/api/volumen/feed.xml"
+    assert_predicate last_response, :ok?
+    assert_equal "application/rss+xml", last_response["Content-Type"]&.split(";")&.first
+    body = last_response.body
+    assert_includes body, "<rss version=\"2.0\">"
+    assert_includes body, "<title>Hello</title>"
+    refute_includes body, "Draft"
+    assert_includes body, "<language>en</language>"
+  end
 end