fix: add next.js 16 compatibility for string nextConfig

[ItsYash1421]

Summary

Fix Next.js 16 compatibility by handling string nextConfig parameters to prevent TypeError during webpack property assignment.

Changes

• Add type check to convert string nextConfig to empty object in compiler's next() function
• Add comprehensive test for Next.js 16 string nextConfig compatibility scenario
• Prevent "Cannot create property 'webpack' on string 'phase-production-build'" error

Testing

Business logic tests added:

• Test validates string nextConfig ('phase-production-build') gets converted to object without throwing TypeError
• Test ensures webpack function is properly assigned to converted config object
• All tests pass locally

Visuals

Required for UI/UX changes:

• Before/after screenshots attached
• Video demo for interactions (< 30s)

Checklist

• Changeset added (if version bump needed)
• Tests cover business logic (not just happy path)
• No breaking changes (or documented below)

Closes #1651