fix: Activity count not incremented for anonymous posts in Learners tab

[rahulkanneri-2u]

Description

The existing stats logic was inconsistently excluding anonymous threads and comments from course stats depending on the backend. For MySQL, anonymous posts were being filtered out from build_course_stats, update_all_users_in_course, and restore_thread/restore_comment operations. For MongoDB, the exclusion only happened in create_thread and delete_thread. This PR aligns both backends: MySQL now counts anonymous posts in all stat operations (matching the intended behaviour), while MongoDB continues to exclude anonymous posts only in create_thread and delete_thread. The e2e test assertion is also corrected to reflect the per-backend expected counts.

Changes Included

• forum/api/threads.py — Updated create_thread() and delete_thread() guards to skip stat updates only for MongoDB + anonymous posts; MySQL now always updates stats regardless of anonymity.
• forum/backends/mysql/api.py — Removed anonymous=False / anonymous_to_peers=False filters from build_course_stats() and update_all_users_in_course() queries; removed the is_anonymous guard from restore_comment() and restore_thread() so restoring anonymous content correctly reverses its stats.
• tests/e2e/test_users.py — Fixed test_build_course_stats_with_anonymous_posts assertion to expect 1 thread for MongoDB and 3 for MySQL, and updated the docstring to reflect actual intent.
• tests/test_backends/test_mysql/test_api.py — Added four new unit tests covering: counting anonymous threads, counting anonymous-to-peers threads in build_course_stats, and stats updates on restore_thread / restore_comment for anonymous content.

Tickets

• COSMO2-895