fix: unique imports in #nitro-internal-virtual/server-handlers-meta

[ZerxZ]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR fixes an issue with duplicate handler imports in the virtual server handlers meta file, as discussed in nuxt/nuxt#29804.

Current Behavior

Currently, when multiple routes point to the same handler, the handler is imported multiple times in the #nitro-internal-virtual/server-handlers-meta virtual module, causing unnecessary code duplication and potential naming conflicts.

Changes Made

• Modified the handler meta generation logic to use unique imports
• Refactored the code to first collect all unique handlers
• Ensured each handler is imported only once with a consistent import identifier
• Maintained the correct mapping between routes and their corresponding handler metadata

Impact

This change:

• Reduces bundle size by eliminating duplicate imports
• Prevents potential naming conflicts in generated code
• Improves code maintainability
• Has no breaking changes to the public API

Example

Before:

import handler1Meta from "handler1?meta";
import handler1Meta from "handler1?meta"; // Duplicate import

After:

import handler1Meta from "handler1?meta"; // Single import

This fix ensures better code generation while maintaining all existing functionality

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.