fix: preserve context for synchronous async expression errors

[svelte-triage-bot]

Summary

Fixes #18824.

• defer clearing the active reaction context when an async expression throws synchronously
• allow the original error to reach the enclosing <svelte:boundary> instead of producing a null.f TypeError
• add a production-client runtime regression test and patch changeset

In production, the awaited expression can compile to a synchronous thunk. Its synchronous-throw path cleared the active context before update_reaction finished, corrupting subsequent error handling. Deferring that cleanup to a microtask keeps the current update valid while preserving the existing asynchronous cleanup behavior.

Tests

• pnpm vitest run packages/svelte/tests/runtime-runes/test.ts -t async-boundary-sync-throw --reporter=dot
• pnpm test runtime-runes
• pnpm test
• pnpm check
• pnpm lint
• pnpm format

[pkg-svelte-dev]

Install the latest version of svelte from 0f87bfa:

pnpm add https://pkg.svelte.dev/svelte/c/0f87bfaafc85a025ed263fe306ed43bac097683f

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18825

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0f87bfa

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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