Single sync option if index is deleted

[MARQAS]

Description of the Change

This PR will address the issue where the Sync screen was showing 2 sync options along with the Last sync info. This will only show the Sync option which is visible during the initial syn.

Closes #2962

How to test the Change

• In WP-CLI run wp elasticpress delete-index --index-name=$index_name --yes
• Visit the Sync page
• Observe that it shows single sync option without last sync info

Changelog Entry

Fixed - Sync page shows a successful previous sync even if the index has been deleted

Credits

Props @MARQAS

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.

[felipeelia]

The approach you have here almost works @MARQAS but if you delete only one index, you get a false positive.
Steps to reproduce:

1. Enable "Post Search" and "Users" features
2. Run a sync and see 2 indices in the Health screen
3. Delete only one index (wp elasticpress delete-index --index-name=elasticpresstest-post-1 --yes)
4. See 2 boxes in the Sync page (it should have only the Full Sync one)

[felipeelia]

get_totals() are not the best way to check what we need @MARQAS. What we will need to do is to compare the results of Command::get_cluster_indexes() and Command::get_index_names(). As we won't be calling methods from the Command class outside the WP_CLI context, we can create similar methods in the Elasticsearch class and start calling those from the Command class and here, in the Sync class.

[felipeelia]

@MARQAS, In addition to my comments. as you've moved the get_index_names and get_cluster_indices methods to the Elasticsearch class, shouldn't that generate a change in Command.php? Thanks!