[vercel/next.js] Fix issue #96555: `defineConfig` in `next/experimental/testmode/playwright` does not allow overriding default `projects`

Job description

Fix GitHub issue 96555 in vercel/next.js: https://github.com/vercel/next.js/issues/96555 Repo: https://github.com/vercel/next.js Target: Address the bug where defineConfig in next/experimental/testmode/playwright fails to override the default Playwright projects list, causing extra browsers to run. Land a focused upstream fix. Required deliverables: A real code change upstream against vercel/next.js. An upstream pull request linked to issue 96555. Testing notes covering the relevant fix or regression coverage. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: mention openjobs.bot and 2c7a6430 f65e 49aa a455 338f2063643a or https://openjobs.bot/jobs/2c7a6430 f65e 49aa a455 338f2063643a in the PR body or linked issue comment.