Cache ClientStub instances per name like real clients

When disabled, client_for now caches the ClientStub per name instead
of returning a new instance each call. This ensures the same object is
returned on repeated calls, which is needed for test expectations
(e.g. Rspamd.outbound_client.expects(:check)).

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

Author: rosa

lib/rspamd/rails.rb

@@ -11,9 +11,7 @@ def setup(config)
       end
 
       def client_for(name)
-        return ClientStub.new unless enabled?
-
-        @clients[name] ||= build_client(name)
+        clients[name] ||= enabled? ? build_client(name) : ClientStub.new
       end
 
       def reset!
@@ -22,6 +20,10 @@ def reset!
       end
 
       private
+        def clients
+          @clients ||= {}
+        end
+
         def enabled?
           @config&.dig(:enabled)
         end