fix: Silently skip prefetching external URLs instead of warning

[Greenheart]

Fix #6463 by automatically skipping prefetching of external URLs instead of warning app developers. This aligns with how Next.js solves the same problem.

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🦋 Changeset detected

Latest commit: 40824bb

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

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@sveltejs/kit	Patch

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[Rich-Harris]

Thank you! I think the error is useful in the case where you're programmatically prefetching. What about changing the behaviour of trigger_prefetch to ignore any links that would normally be ignored? (Might need to abstract some logic out from the click handler to avoid repetition)

[Greenheart]

Thanks for the feedback! That sounds like a good way forward. When app devs call prefetch() there's a clear intention and the warning would be helpful.

I'll explore this further in the coming days. This would be a fun contribution to make!

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[Greenheart]

Update:

Prefetching never happens for external URLs.

The difference is that programmatic calls to prefetch() will now throw the same error as before, while trigger_prefetch that is activated by data-sveltekit-prefetch silently fails just like the previous commit.

Remaning TODOs

• I get two failing tests locally which didn't fail before. Any ideas what might have caused this? Perhaps because we're now inconsistent and throwing errors in one case but not in other cases? Or this might be caused by test flakiness of some kind.

@sveltejs/kit:test:     [chromium-dev] › test/client.test.js:309:2 › Errors › Root error falls back to error.html ======
@sveltejs/kit:test:     [chromium-dev] › test/client.test.js:360:2 › Load › accessing url.hash from load errors and suggests using page store

[dummdidumm]

CI succeeded and I don't get these test failures locally, so I'd say we are good - thank you 👍