test: add sitemap.xml endpoint test

Verify content type, urlset structure, post URL presence, and draft exclusion.

test/server_test.rb

@@ -109,4 +109,14 @@ class ServerTest < Minitest::Test
     assert_equal 404, last_response.status
     assert_equal "not_found", JSON.parse(last_response.body)["error"]
   end
+
+  def test_sitemap_endpoint
+    get "/api/volumen/sitemap.xml"
+    assert_predicate last_response, :ok?
+    assert_equal "application/xml", last_response["Content-Type"]&.split(";")&.first
+    body = last_response.body
+    assert_includes body, "<urlset xmlns=\"http://www.sitemaps.org/schemas/sitemap/0.9\">"
+    assert_includes body, "<loc>https://example.com/hello</loc>"
+    refute_includes body, "draft"
+  end
 end