fix: fall back when service worker registration fulfills with undefined

[MFA-G]

The snippet's register().then() success callback reads swRegistration.active unguarded. Some environments fulfill register() with undefined instead of rejecting it. Playwright's serviceWorkers: 'block' does this: I checked in Chromium, and navigator.serviceWorker.register(...) resolves to undefined. The read then throws TypeError: Cannot read properties of undefined (reading 'active') as an unhandled rejection. Partytown only recovers once fallbackTimeout fires (9999 ms by default).

Reported downstream in withastro/astro#18171, which shows this in production on real Chrome/Android user agents as well as HeadlessChrome.

Change

• A missing registration now calls fallback() right away, the same as a rejected register() (fix: fall back partytown scripts added after the fallback ran #751).

Tests

• tests/unit/snippet.spec.ts: new case where register() fulfills with undefined. It asserts the partytown script falls back to the main thread. Before the fix it fails with the TypeError.
• tests/platform/fallback: this test already uses serviceWorkers: 'block'. It passed only because of the 1 s fallbackTimeout while the error went unnoticed. It now also asserts there are no page errors. That assertion fails on main with "Cannot read properties of undefined (reading 'active')" and passes with this change.
• pnpm test.unit: 52/52. playwright test tests/platform --browser=chromium: 44 passed. fmt.check passes.
• Added a patch changeset.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9b3fb1c

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

This PR includes changesets to release 1 package

Name	Type
@qwik.dev/partytown	Patch

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[gioboa]

Great fix 👏 Thanks @MFA-G