feat: implement bulk delete for MySQL database

[Alam-2U]

Description

Bulk delete functionality is currently implemented only for the MongoDB backend, while the corresponding implementation for the MySQL backend is still pending. As part of the migration from MongoDB to MySQL, it is essential to implement bulk delete support in the MySQL backend to ensure feature parity and consistent behavior across both systems.

Ticket:
COSMO2-885

Related ticket
edx/edx-platform#218

[Copilot]

Pull request overview

Implements bulk delete and count APIs for user-authored forum content to support feature parity during the MongoDB → MySQL migration (COSMO2-885).

Changes:

• Added public API functions to count and bulk-delete a user’s threads/comments across multiple courses.
• Implemented corresponding backend methods for MongoDB and MySQL.
• Updated forum.api exports and bumped package version.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
forum/backends/mysql/api.py	Adds MySQL implementations for counting and bulk soft-deleting user threads/comments.
forum/backends/mongodb/api.py	Adds Mongo backend API wrappers for new count/bulk-delete operations.
forum/backends/mongodb/threads.py	Adds Mongo implementation for bulk deleting user threads via forum API.
forum/backends/mongodb/comments.py	Adds Mongo implementation for bulk deleting user comments via forum API.
forum/api/threads.py	Exposes get_user_threads_count and delete_user_threads at the API layer.
forum/api/comments.py	Exposes get_user_comment_count and delete_user_comments at the API layer.
forum/api/__init__.py	Re-exports the new API functions in __all__.
forum/__init__.py	Bumps version from 0.4.6 → 0.4.7.

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