[Bug]: altis_search_index_version option could end up in `notoptions` because of a race condition

[jerico]

Bug Description

Race condition can occur when multiple requests simultaneously try to trigger the option_update/add_option code to bump altis_search_index_version from 2 to 3 in https://github.com/humanmade/altis-enhanced-search/blob/master/inc/namespace.php#L566-L573. Option ends up in notoptions cache despite existing in database as version 3.

When this happens every subsequent request triggers:

• get_option('altis_search_index_version', null) returns null (due to notoptions)
• Code in filter_index_name() checks if version is 2, then calls set_index_version(3)
• This causes update_option() which attempts INSERT (since cache thinks option doesn't exist) and make all the queries go to the writer db

Steps to Reproduce and Code Sample

Enable ElasticSearch on a high traffic site

Expected Behaviour

The altis_search_index_version option should remain properly cached and not fall into notoptions when it exists in the database. Don’t update the DB on viewer traffic.

Additional Info

More info: https://hmn.slack.com/archives/C0A2F2VC58A/p1765305934093979