Fix targetless workflow test for pre/post processing framework

The test was failing because it didn't stub the output_manager method
that's now being called in workflow_runner.rb for the pre/post processing
feature.

Author: obie

Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: .
   specs:
     roast-ai (0.2.1)
-      activesupport (~> 7.0)
+      activesupport (>= 7.0)
       cli-ui
       diff-lcs (~> 1.5)
       faraday-retry

test/roast/workflow/targetless_workflow_test.rb

@@ -44,6 +44,10 @@ def test_executes_workflow_without_a_target
       workflow.stubs(:respond_to?).with(:session_name).returns(true)
       workflow.stubs(:respond_to?).with(:final_output).returns(true)
 
+      # Stub output_manager for the pre/post processing code
+      output_manager = mock("output_manager")
+      workflow.stubs(:output_manager).returns(output_manager)
+
       Roast::Workflow::BaseWorkflow.expects(:new).with(
         nil,
         has_entries(name: instance_of(String), context_path: instance_of(String)),