style(test): add explicit :ok return in maybe_warn_about_dsn_override

Add explicit :ok after Logger.warning call so the return value is
clearly visible rather than relying on Logger.warning returning :ok.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: solnic

lib/sentry/test/registry.ex

@@ -76,6 +76,8 @@ defmodule Sentry.Test.Registry do
         unset it for test runs or set :dsn explicitly in your test config.\
         """)
 
+        :ok
+
       nil ->
         :ok