feat(core): agnostic background task UI with CompletionBehavior (#22740)

Co-authored-by: mkorwel <matt.korwel@gmail.com>

packages/cli/src/ui/App.test.tsx

@@ -88,7 +88,7 @@ describe('App', () => {
       defaultText: 'Mock Banner Text',
       warningText: '',
     },
-    backgroundShells: new Map(),
+    backgroundTasks: new Map(),
   };
 
   it('should render main content and composer when not quitting', async () => {