fix: ensure <svelte:boundary> properly removes error content in production mode

[zeroberry]

fixes #15674

When DEV=false, the should_rethrow_error check wasn't being performed, which led to errors during the initial mount not triggering destroy_effect. This prevented proper removal of elements inside the boundary. I belive this issue relates to commit 10897ac.

To address this, I've moved the DEV-specific logic into an explicit if block to distinguishes between DEV and non-DEV behaviors, ensuring consistency across both scenarios.

I couldn't run test with DEV=false because DEV is controlled by the NODE_ENV=production via esm-env. So I've added a test case to simulate a similar condition by creating an error scenario where component_context is null.

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[changeset-bot]

🦋 Changeset detected

Latest commit: d65d85b

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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[Rich-Harris]

this is great, thank you!

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15793