I18N: Add wp_search_term filter to allow per-term normalization

[SainathPoojary]

Trac ticket: #25585

Adds a new wp_search_term filter to WP_Query::parse_search_terms(). This allows plugins to normalize individual search terms (e.g., stripping Arabic diacritics/Hamza variants) or completely remove specific terms from a search query by returning an empty string.

Testing Instructions:

1. Run PHPUnit tests: npm run test:php -- tests/phpunit/tests/query/search.php
2. Verify all tests pass.

[github-actions]

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