Filter popular categories before applying the result limit

[RabbiIslamRony]

PR Type

What kind of change does this PR introduce?

• Bugfix
• Security fix
• Improvement
• New Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Text changes
• Other... Please describe:

Description

How to reproduce the issue or how to test the changes

1. Create two directory types and assign more high-count root categories to the second directory than the configured Popular Categories limit.
2. Assign indexed-only, legacy-only, and dual-key root categories to the first directory, and add an empty category plus a high-count child category.
3. Open the first directory's search page and confirm the configured number of non-empty root categories is returned in count-descending order, without duplicates or categories from the second directory.
4. Inspect the term query and confirm directory meta and tt.parent = 0 are applied before the bounded LIMIT clause.

The directory constraint now runs inside the term query before the result limit. It supports the indexed _directory_type_<id> key and the legacy serialized _directory_type key, while preserving root-only, non-empty, count-descending behavior.

Any linked issues

Fixes #820

Checklist

• My code follows the WordPress coding standards

[RabbiIslamRony]

The reported PHPCS failure occurs in the inherited pull_request_target workflow before source checkout; the PHPCS step never runs. PR #2945 replaces that unsafe workflow with the fork-safe pull_request workflow. The changed PHP file passes the repository rules locally with zero errors.