Feature: Duplicate collection with all requests

[AkashBhuiyan]

Description

Add API endpoint to duplicate a collection including all its requests.

API Endpoint

POST /api/v1/collections/{collectionId}/duplicate
{
  "newName": "Collection Copy"
}

Requirements

• Copy collection metadata
• Copy all requests
• Copy request headers and params
• Generate new IDs
• Handle name conflicts

Labels

enhancement, good first issue