UI: Fix Gantt queued tooltip end date

[YinkaMetrics]

The Gantt queued and scheduled segments reused the hovered segment end as the tooltip end date. For queued segments, that made the tooltip show the queue end/start time as the task end date instead of the actual task end.

This keeps the effective task end date on every Gantt segment and uses it when building tooltip summaries, so queued/scheduled segment tooltips stay consistent with the execution segment.

closes: #68174

Validation:

• pnpm vitest run src/layouts/Details/Gantt/utils.test.ts
• pnpm prettier --check src/layouts/Details/Gantt/GanttTimeline.tsx src/layouts/Details/Gantt/utils.ts src/layouts/Details/Gantt/utils.test.ts
• pnpm eslint --quiet src/layouts/Details/Gantt/GanttTimeline.tsx src/layouts/Details/Gantt/utils.ts src/layouts/Details/Gantt/utils.test.ts
• pnpm tsc --p tsconfig.app.json --noEmit
• prek commit hooks

Note: I did not attach screenshots because this is a tooltip timestamp data fix with regression coverage and no layout/styling change.

---

Was generative AI tooling used to co-author this PR?

• Yes: OpenAI Codex

Generated-by: OpenAI Codex following the guidelines

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example Dag that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[pierrejeambrun]

Thanks for opening the PR.

Closing in favor of #68570 which was just merged.