Guard Instant Results facets against invalid taxonomy values - #4255

Description of the Change

This PR prevents a fatal error in Instant Results taxonomy facets when the ep_facet_include_taxonomies filter returns unexpected values (e.g., a taxonomy slug string such as internal_tags rather than a WP_Taxonomy object).

InstantResults::get_facets() currently assumes each filtered taxonomy entry is a WP_Taxonomy object and passes it directly to get_taxonomy_labels(). If a third-party integration appends a slug string, WordPress core fatals when it tries to treat a string as an object (e.g., Attempt to assign property "labels" on string in wp-includes/taxonomy.php).

This change adds a defensive guard in the taxonomy facets loop:

• If the value is a string slug, normalize it via get_taxonomy( $slug )
• If the value cannot be resolved to WP_Taxonomy, skip it

This prevents wp-admin lockouts while preserving existing behavior for correctly shaped data.

Closes #4254

How to test the Change

1. Install and activate ElasticPress 5.3.2 and enable Instant Results.
2. Add the reproduction filter from issue BUG: Fatal error in Instant Results facets when ep_facet_include_taxonomies contains a taxonomy slug string (e.g. internal_tags) #4254 (appends internal_tags as a string via ep_facet_include_taxonomies).
3. Log in to wp-admin and visit:
   • /wp-admin/
   • /wp-admin/admin.php?page=elasticpress
4. Confirm both pages load successfully and no fatal error occurs.

Verified on: WordPress 6.9; PHP 8.4.

Changelog Entry

Fixed - Prevent fatal error in Instant Results facets when ep_facet_include_taxonomies returns non-WP_Taxonomy values.

Credits

Props @laraib15

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.