Fix TypeScript errors in "Configuration" & "Routing and navigation" code snippets

[ArmandPhilippot]

Description (required)

Fixes TypeScript errors in "Configuration" and "Routing and navigation" pages:

• prefetch.mdx
  • document.getElementById can be null
  • forEach doesn't work with HTMLCollectionOf
  • getAttribute can return null
• internationalization.mdx
  • ctx.url is a URL, startsWith is not available
  • missing prop in redirectToDefaultLocale (we should pass ctx)
  • useless async
• middleware.mdx
  • missing import
• typescript.mdx:
  • showing as Params when satisfies GetStaticPaths is used is misleading: we don't need a cast here

To help review and explain my changes I made a commit per code snippet (with a few exceptions where the changes were the same), with a short explanation of why in the commit message. But, I also repeat the explanation above. Choose what is easier for you!

If you're interested in the "why," here's my reasoning:

Code snippets should work with minimal friction for users: TypeScript red squiggles or missing imports provide bad DX, especially for people new with web dev. I guess this can help AI too now.

Also, since Astro v5, the default Typescript preset is strict instead of base. This means TypeScript is now reporting errors for some code snippets that could work before. We had some reports before, we fixed them. We still get new reports from time to time, so I'm starting to check all our code snippets.

re: formatting changes
In the core repo they tend to use single quotes, but in a new project without custom configuration (in VSCode, with the Astro extension enabled), this is automatically formatted with double quotes: I think minimal visual changes for the user are a good thing. So, I haven't undone the formatting changes made when correcting the code snippets.

kinda related to #10073

Related issues & labels (optional)

• Suggested label: code snippet update

[astrobot-houston]

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Tracked Files

File	Note
en/guides/internationalization.mdx	Source changed, localizations will be marked as outdated.
en/guides/middleware.mdx	Source changed, localizations will be marked as outdated.
en/guides/prefetch.mdx	Source changed, localizations will be marked as outdated.
en/guides/typescript.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[github-actions]

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[ArmandPhilippot]

This is ready for review... but I can see this is a lot to review at once. I'll split it!

This is now focused on "Configuration" & "Routing and navigation" code snippets, follow-up PRs:

• Fix TypeScript errors in "Build your UI" code snippets #14090
• Fix TypeScript errors in "Add content to your site" code snippets #14091
• Fix TypeScript errors in "Server rendering" code snippets #14092

Well, the PR is smaller but the description is now bigger. 😅 Feel free to ask for more details!

[ArmandPhilippot]

I extracted a few changes in follow-up PRs to ease the review:

• Fix TypeScript errors in view-transitions.mdx code snippets #14182
• Fix TypeScript errors in endpoints.mdx code snippets #14181
• Fix TypeScript errors in guides/routing.mdx code snippets #14180