fix: ensure async deriveds always get dependencies from thennable

[dummdidumm]

When an async derived already has a previous promise that is still pending, we were not accessing the then property of the new promise. If that property access causes signals to be read, that meant that those dependencies were lost and as such the derived wouldn't rerun anymore when it should. The fix is to make sure to always access the thennable.

Discovered this while working on the query.stream PR (it didn't update in some cases and this is the reason why)

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Playground

pnpm add https://pkg.pr.new/svelte@16672

[DePasqualeOrg]

Something in svelte@5.38.5 broke top-level await for me. Data loaded with top-level await silently fails to load. 5.38.3 is the last version in which this still works.

[Antonio-Bennett]

@DePasqualeOrg I'd say open a new issue tagging this PR with a minimum reproduction this might go unnoticed since it's on a merged PR