Simplify Plotly testing setup

[cotedav]

Summary

• remove the Vitest alias for plotly.js-dist-min so tests resolve the installed package while using the global mock
• drop the redundant Plotly stub file that was only used by the alias

Testing

• npm run ci

---

Codex Task

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.