Green the jsdom lane for the composer-seat ResizeObserver and new StatsLine format

The seat's height publisher needs a ResizeObserver stub in every spec that
renders ConversationRoot (jsdom has none), and the two branch-tail StatsLine
assertions move to the grouped detail-row output.

packages/client/ui-conversation/tests/chat-branch-tails.spec.tsx

@@ -216,6 +216,6 @@ describe('small branch tails', () => {
     const view = render(
       <StatsLine useSession={bindSnapshotSelector(source) as unknown as StatsLineProps['useSession']} />,
     )
-    expect(view.getByText('10 tokens · 1 turns · 1 steps')).toBeTruthy()
+    expect(view.container.textContent).toBe('1 turns · 1 steps|Input 0 tok · Output 10 tok')
   })
 })