fix: abort deriveds own AbortSignal when it disconnects

[gyzerok]

Fixes #18301

The issue explains it all. Even though no maintainer verified it as a bug this PR is simple enough, that reviewing it might be easier instead.

If it's not considered a bug we can at least keep the test changes (with modifications) to otherwise "enforce" current behavior.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2bd7dd7

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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svelte	Patch

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