fix(public-assets): do not shadow paths that share prefix

[danielroe]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Encountered this as a side effect of replicating another bug.

Registering public asset dir with a base URL of, say, /custom will lead to paths like /custom-thing also being handled by the same handler (and returning a hard 404 if fallthrough is disabled). This is because we do not test if the path begins with /custom/ but with /custom.

Let me know if you need me to provide a reproduction. I think the problem and fix are straightforward to see, but happy to do so!

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.