This PR is a follow up on #10009 which ensured that the fetch function from the load event would call window.fetch so that libraries (like Sentry but there are many others) can patch it. Unfortunately, I missed back then that there's another kind of fetch request made by SvelteKit: Whenever a user navigates to a route that contains a server-load function, a request is made to fetch the server-load data.

This PR ensures that this server-load data request is also patchable by basically using the same mechanism introduced in #10009. I also added a test that would prior to this change fail.

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