fix(test-runner-core): await browser cleanup in watch mode

[raashish1601]

What I did

Watch-mode shutdown puts browser cleanup promises into a shadowed local array, so stop() can resolve and emit stopped while a browser is still closing. The CLI exits on that event.

Removed the shadowed declaration so shutdown awaits both server and browser cleanup. Added a regression that holds browser cleanup open after the real server has stopped, plus a patch changeset.

Validation:

• The new regression fails on upstream because stopped fires before browser cleanup finishes.
• npm run build passes.
• All 23 core Node tests pass with one worker.
• Full ESLint, changed-file Prettier, repository-wide Prettier, and the normal precommit hook pass.

These local results cover the core package; broader repository CI has not been run locally.

Developed with OpenAI Codex assistance.

[changeset-bot]

🦋 Changeset detected

Latest commit: d18c4b6

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

This PR includes changesets to release 1 package

Name	Type
@web/test-runner-core	Patch

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