fix/4191: Search template generation when Post Search feature is disabled

[Sidsector9]

Description of the Change

Instant Results failed to generate a search template when the Post Search feature was disabled. The Post indexable was never activated, so the query in get_search_template() fell back to MySQL instead of Elasticsearch and no template was captured.

Closes #4191

How to test the Change

Changelog Entry

Fixed - Search template generation when Post Search feature is disabled

Credits

Props @burhandodhy @Sidsector9

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.

[felipeelia]

Wouldn't it make more sense to make Instant Results dependent on the Post Search feature?

[Sidsector9]

@felipeelia my implementation was based on the absence of checks for Post Search, for both Autosuggest and Instant Results. It will make sense if we make both features dependent on Post Search feature and add a message under each result that "This feature requires Post Search to be enabled". Something like this: