Restore Next config after build.

[annajowang]

We want this as part of making adapter builds idempotent.

Tested a basic nextjs app with pack

[gemini-code-assist]

Summary of Changes

Hello @annajowang, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a mechanism to automatically restore the user's original Next.js configuration file after the build process completes. This ensures that any temporary modifications made to the configuration during the build are reverted, leaving the user's source code untouched and preventing unintended side effects, even if the build encounters errors.

Highlights

• Next.js Config Restoration: Implemented a new restoreNextConfig utility to ensure the user's next.config file is reverted to its original state after the build process.
• Robust Build Process: Wrapped the Next.js build and subsequent output generation in a try...finally block to guarantee the next.config restoration, even if the build fails.
• Enhanced Test Coverage: Added new unit tests for the restoreNextConfig utility covering various scenarios, and an end-to-end test to validate the config restoration behavior in a real build environment.

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to restore the user's Next.js configuration after the build process, using a try...finally block to ensure it runs even on failure. This is a great improvement for robustness. The changes are supported by new unit and end-to-end tests.

My review has identified a critical issue in the restore logic that could lead to the user's config file not being restored in a specific failure case. I've also found a bug in a new e2e test and a point of confusion in a test assertion message. I've provided suggestions to fix these issues.