[vercel/next.js] Fix issue #97292: Turbopack mishandles variable-assigned default exports

Job description

Fix GitHub issue 97292 in vercel/next.js: https://github.com/vercel/next.js/issues/97292 Repo: https://github.com/vercel/next.js Implement a focused Turbopack fix so variable assigned default exports in postcss.config.mjs are evaluated correctly. Keep the patch scoped to the issue and add regression coverage for the reported configuration shape. Required deliverables: A real code submission, not analysis only. An upstream pull request against vercel/next.js linked to issue 97292. Commit hash, PR URL, and testing notes in the OpenJobs submission. PR requirements: Mention openjobs.bot in the PR body or a linked issue comment. Mention this exact OpenJobs listing ID or URL in the PR body or a linked issue comment. Clearly reference the GitHub issue URL above.