chore: refine Rails.root.join stub in spec

spec/lib/captain/base_task_service_spec.rb

@@ -280,7 +280,10 @@ RSpec.describe Captain::BaseTaskService do
 
   describe '#prompt_from_file' do
     it 'reads prompt from file' do
-      allow(Rails.root).to receive(:join).and_return(instance_double(Pathname, read: 'Test prompt content'))
+      allow(Rails.root).to receive(:join).and_call_original
+      allow(Rails.root).to receive(:join)
+        .with('lib/integrations/openai/openai_prompts', 'test.liquid')
+        .and_return(instance_double(Pathname, read: 'Test prompt content'))
       expect(service.send(:prompt_from_file, 'test')).to eq('Test prompt content')
     end
   end