fix(dart_frog_cli): avoid overwriting pubspec_overrides.yaml in dart_frog build

[felangel]

Status

READY

Description

• fix(dart_frog_cli): avoid overwriting pubspec_overrides.yaml in dart_frog build
  • createExternalPackagesFolder assumed an existing pubspec_overrides.yaml did not exist which isn't a valid assumption
  • also refactors the prod server hook to reuse existing helpers rather than reimplementing them inline

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore