The file manager spec gets a directory nobody else can be in

Twice in a long session the suite came back with one failure in this file, and
neither run could be reproduced afterwards — not by seed, not by repetition.
The one thing this spec has that the other 39 examples do not is a fixed path
under `tmp/`, which anything else on the machine can also be inside.

`Dir.mktmpdir` instead, so two runs cannot see each other's folders at all. If
a failure survives this, it is about the code and not about the directory.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/api/spec/requests/admin_files_spec.rb

@@ -1,14 +1,14 @@
 require "rails_helper"
 require "warden/test/helpers"
+require "tmpdir"
 
 RSpec.describe "Admin file manager", type: :request do
   include Warden::Test::Helpers
 
   let(:admin) { AdminUser.create!(email: "files-spec@example.org", password: "password123") }
-  let(:container) { Rails.root.join("tmp/files-spec").to_s }
+  let(:container) { Dir.mktmpdir("files-spec") }
 
   before do
-    FileUtils.rm_rf(container)
     FileUtils.mkdir_p(File.join(container, "mygame-1.0"))
     File.write(File.join(container, "mygame-1.0.zip"), "zipdata")
     allow(WarpEngine.config).to receive(:file_container_path).and_return(container)