Add regression test for TUI detail scroll preservation

Non-blocking follow-up from PR #1199 review for WL-0MN8QL3AO0008ZW3. Add an automated test that verifies the detail pane retains scroll position when the same work item is re-rendered, and resets scroll only when navigating to a different item. Include clear setup and assertions so this regression is caught in CI.\n\nrelated-to:WL-0MN8QL3AO0008ZW3