test(gantt): cover updateTaskRecord task bar refresh

[fangsmile]

Summary

• add regression coverage proving updateTaskRecord refreshes the task record and rendered bar geometry
• add a runnable Gantt example for the final-row date update reported in [Bug] 调用gantt图的updateTaskRecord方法修改任务的起始、终止时间，gantt 无变化 #4534
• document the already-correct behavior on current develop and protect it from regression

Closes #4534

Test plan

• rushx test --runInBand gantt-update-task-record.test.ts
• rushx test --runInBand (9 suites, 27 tests)
• rushx compile
• ESLint on all changed files
• rush test --only tag:package via the pre-push hook
• Browser demo: last task moves from x=320, width=120 to x=440, width=200
• Bugserver case: https://bugserver.cn.goofy.app/case?product=VTable&fileid=6aaf5afa49fc4f005c341b50&version=1.0.0.4551

No changeset is included because this PR changes only tests and examples.

🤖 Generated with Claude Code

[fangsmile]

Closing this PR because the added regression case does not reproduce #4534. The same delayed post-render update passes on the reported v1.20.1 release, and the linked CodeSandbox is no longer available (404/private), so this test-only change cannot be tied to a verified fix. We should reopen with an updated reproduction that fails before a concrete code change.