[url-check] New broken URLs detected

[github-actions]

The weekly URL audit found 3 broken URL(s) in repo that are not in .url_check_allowlist.txt.

If these are genuinely-broken-but-acceptable (paywalled, dead-external, etc.), append them to .url_check_allowlist.txt to grandfather them. Otherwise fix the references in-repo.

https://colab.research.google.com/github/PyAutoLabs/HowToGalaxy/blob/2026.5.29.4/notebooks/chapter_4_pixelizations/tutorial_6_model_fit.ipynb

• status: HTTP 404
• referenced from 1 site(s):
  • docs/howtogalaxy/chapter_4_pixelizations.md:19

https://colab.research.google.com/github/PyAutoLabs/HowToGalaxy/blob/2026.5.29.4/notebooks/chapter_optional/tutorial_mass_profiles.ipynb

• status: HTTP 404
• referenced from 1 site(s):
  • docs/howtogalaxy/chapter_optional.md:7

https://colab.research.google.com/github/PyAutoLabs/HowToGalaxy/blob/2026.5.29.4/notebooks/chapter_optional/tutorial_sub_grids.ipynb

• status: HTTP 404
• referenced from 1 site(s):
  • docs/howtogalaxy/chapter_optional.md:10

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