Fix TypeScript errors in endpoints.mdx code snippets

[ArmandPhilippot]

Description (required)

Follow-up of #14089. TL;DR: Improve UX by preventing the appearance of red squiggles (TS) and visual changes (formatting with Astro extension) when the user copies and pastes the code snippet.

Fixes code snippets in guides/endpoints.mdx:

• params.id is string | undefined and undefined "cannot be used as an index type". If we narrow the type to string (using APIRoute type parameters), TypeScript complains because a string can't be used for index access... Number() might not be the best (NaN) but it's the easier fix to make it TS friendly.
• without satisfies APIRoute, TypeScript complains about the destructured properties.
• TypeScript complains about Buffer and, unlike process.env, there is no clues in the editor to fix this, adding a comment might be helpful
• missing import

Related issues & labels (optional)

• Suggested label: code snippet update

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