Add test for ErrorHTML fallback render function

Added test to cover the fallback render/2 function that handles
error codes without custom templates (401, 403, 503, etc.).

This improves ErrorHTML coverage from 0% to 100%.

test/towerops_web/controllers/error_html_test.exs

@@ -16,4 +16,16 @@ defmodule ToweropsWeb.ErrorHTMLTest do
     assert html =~ "500"
     assert html =~ "Something went wrong on our end"
   end
+
+  test "renders fallback status message for unknown templates" do
+    # Test the fallback render/2 function for error codes without custom templates
+    result = ToweropsWeb.ErrorHTML.render("503.html", %{})
+    assert result == "Service Unavailable"
+
+    result = ToweropsWeb.ErrorHTML.render("401.html", %{})
+    assert result == "Unauthorized"
+
+    result = ToweropsWeb.ErrorHTML.render("403.html", %{})
+    assert result == "Forbidden"
+  end
 end