fix: prevent isr routes from handling remote function calls (#15085)

[arichardsmith]

This PR fixes #15085 by adding a routing table entry to make sure /_app/remote/* routes are handled on Vercel by a non-ISR route. It also symlinks /_app/remote so remote function calls are shown under this path in Vercel's observability dashboard.

This addresses an issue that happened when a /[...rest] route had ISR settings. The payload query param would be stripped out, meaning the remote function parameters weren't passed through. It also meant the result of remote function queries would be cached as per the routes ISR settings.

One thing to consider for the future; we only emit the route table entry if remote functions are enabled. This is under kit.experimental in the config, so when remote functions become non-experimental, the check will need to be updated. Ideally the builder object would expose something like a hasRemoteFunctions flag or function.

I didn't see any tests for the adapter output, so I haven't added any tests for this. But I have tested my repro against the changed adapter and it works.

@elliott-with-the-longest-name-on-github for a check when you have time!

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🦋 Changeset detected

Latest commit: 0728790

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@sveltejs/adapter-vercel	Patch

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[arichardsmith]

I realised I didn't mention the change to how remote functions show in observability in the changeset. This might be the more visible result from this PR, so is it worth also adding a changeset for that?

[elliott-with-the-longest-name-on-github]

Sure, why not -- let's add a feat: patch calling out the observability improvements. I'm testing this today, hopefully will be able to merge soon.

[arichardsmith]

Thanks! I've added the extra changeset and made sure it's rebased on main, so I think everything's good to go if you are happy with the changes.

[elliott-with-the-longest-name-on-github]

I've finally gotten around to reviewing this and it looks like it does indeed fix the issue... but there's a weird issue where the [...rest] with the ISR config is capturing requests for favicon.png, which I think is not related to this but I need to make sure -- either way will plan on merging this tomorrow with a few minor adjustments

[elliott-with-the-longest-name-on-github]

LGTM, the favicon "issue" is just that some web browser (maybe mine, maybe the one taking the site screenshot after deployment) is looking for favicon.png, which doesn't exist, and therefore is handled correctly by [...rest]

[arichardsmith]

Thanks for the guidance and checking the PR over!