feat: add course translation feature

[ahmed-arb]

Meta Translation Support for edX Platform

This PR adds backend support for the meta-translation feature, enabling course authors to create and manage translated versions of courses. The changes include API enhancements, serializer updates, and proper handling of translation context across various Studio operations.

📁 File Changes

Core Module Store

• xmodule/modulestore/mixed.py
  • Added logic to properly handle deletion of translated courses in the modulestore

REST API Serializers

• cms/djangoapps/contentstore/rest_api/serializers/common.py

  • Added translation context field to common serializer base

• cms/djangoapps/contentstore/rest_api/v1/serializers/settings.py

  • Added is_destination_course field to course settings serializer

• cms/djangoapps/contentstore/rest_api/v1/serializers/vertical_block.py

  • Implemented is_destination_block field in vertical block children serializer to identify translated blocks
  • Enhanced serializer with translation context for course outline page

• cms/djangoapps/contentstore/rest_api/v1/serializers/course_index.py

  • Added translation metadata to course index serializer for outline page API

• cms/djangoapps/contentstore/rest_api/v1/serializers/home.py

  • Fixed missing translation information (base/translated course status) in Studio home API response

• cms/djangoapps/contentstore/rest_api/v1/serializers/course_rerun.py

  • Enhanced course rerun serializer with translation metadata to support creating translated course reruns

• cms/djangoapps/contentstore/rest_api/v2/serializers/home.py

  • Updated v2 home serializer with translation support for course listing

REST API Views

• cms/djangoapps/contentstore/rest_api/v1/views/settings.py

  • Updated settings view to include destination course flag in API response

• cms/djangoapps/contentstore/rest_api/v1/views/course_details.py

  • Migrated translation-aware logic to course details API view

• cms/djangoapps/contentstore/rest_api/v2/views/home.py

  • Modified v2 home view to properly handle and send translation context

Core Views

• cms/djangoapps/contentstore/views/course.py

  • Refactored course view to send translation context in appropriate API endpoints
  • Enhanced course operations with comprehensive translation context handling
  • Updated course rerun functionality to preserve translation relationships

• cms/djangoapps/contentstore/views/preview.py

  • Added translation context to XBlock preview rendering for proper content display in translated courses

Utilities & Handlers

• cms/djangoapps/contentstore/utils.py

  • Added utility functions for translation context management and validation
  • Resolved error when saving course details by fixing translation context handling
  • Enhanced with helper methods for checking and managing translation relationships

• cms/djangoapps/contentstore/tasks.py

  • Updated async tasks to handle translation context properly during background operations

• cms/djangoapps/contentstore/xblock_storage_handlers/view_handlers.py

  • Updated XBlock storage handlers to preserve translation relationships during content operations
  • Ensures translated blocks maintain proper linkage to base course blocks

🎯 Key Features

• ✅ Translation-aware course operations: All course CRUD operations now respect translation context
• ✅ API enhancements: Added is_destination_course and is_destination_block flags throughout REST APIs
• ✅ Course rerun support: Create translated course reruns with proper metadata
• ✅ Preview support: XBlock previews now render with correct translation context
• ✅ Deletion handling: Safe deletion of translated courses with proper cleanup

🧪 Testing Recommendations

This PR should be tested with:

1. Creating translated course reruns
2. Viewing course outline with translated/base course flags
3. Editing content in translated courses
4. Deleting translated courses
5. Previewing XBlocks in translated courses
6. Studio home displaying translation metadata correctly

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Related issue: #555