[#157] Fix inverted hash comparison in filter_indexed_files causing full file reindex

[tsmilan]

Summary

There was a one-character typo != instead of == in the filter_indexed_files() when comparing the content hash in the $unchanged boolean. This meant unchanged files evaluated to $unchanged = false and were queued for re-indexing on every incremental run, even when nothing had changed.

Testing Instructions

1. Create a test course
2. Create an assignment (or other module) and attach multiple files
3. Navigate to Site administration > Server > Tasks > Scheduled task > Global search indexing (core\task\search_index_task)
   and run the indexer
4. Verify that your file has been indexed successfully
5. Update your assignment activity (e.g. title or description) and re-run the search_index_task
6. Check the cron logs and verify that the file is being re-indexed again

Fixes #157

[dmitriim]

@tsmilan can you please bump a version?

[tsmilan]

Thanks @dmitriim I've added a version bump