fix: flush when pending boundaries resolve

[Rich-Harris]

This fixes a somewhat nasty bug that prevents effects (such as attachments) running inside newly-resolved pending boundaries under some circumstances.

As the comment suggests, it feels like a bit of a kludge, but for now we need it.

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[changeset-bot]

🦋 Changeset detected

Latest commit: dfe4f43

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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Playground

pnpm add https://pkg.pr.new/svelte@16897

[PatrickG]

Fixes #16810