fix/759: Add background processing for Embeddings Classification

[Sidsector9]

Description of the Change

This PR implements Action Scheduler to perform Embeddings Classification with 20 terms per job. (The count is opinionated, open to change it).

Closes #759

How to test the Change

1. Visit /wp-admin/tools.php?page=classifai&tab=language_processing
2. Click save.
3. Observe the notice "OpenAI Embeddings: Classification is in progress in the background."
4. Wait until message updates to "OpenAI Embeddings: Classification completed."
5. Verify the embeddings generated in the wp_termmeta table.

Changelog Entry

Added - classifai_openai_embeddings_max_terms filter hook to change the number of terms to process during Embeddings classification per job. Changed - Embeddings classification now uses background processing to improve performance.

Credits

Props @dkotter @Sidsector9

Checklist:

• [ ] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.

[Sidsector9]

@dkotter sorry for the delay, I couldn't find time to pick this up. I'll work on Azure and revert soon for review.