fix: Don't crash on hydratable serialization failures

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

This one was fun to track down. Closes #17313.

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Latest commit: ac41805

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Playground

pnpm add https://pkg.pr.new/svelte@17315

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

I did verify that this crashes the test process prior to this PR -- it ends up with that super unhelpful "some test somewhere had a rejected promise" error. So we're all good now, for sure, and this will catch regressions.

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

As always all this recursive promise stuff is brain-bending, but I think we just had indirection here in the first place. There's no reason we need to actually return the string from this promise, we can just replace the placeholders inline.

[kylejrp]

Looks great to me! Fixes the problem I was having entirely. The exception bubbles up to Fastify's global handler now. Thanks again for the quick fix! (And for correcting my faulty explanation haha 😅)

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

Thanks for the report! The bug was pretty difficult to figure out 😅 Promises are wild