Gutenberg_REST_Posts_Controller_6_7: Support exact search in the REST API posts endpoint.

[david-binda]

What?

Introduces the changes from https://core.trac.wordpress.org/changeset/59034/ to Gutenberg_REST_Posts_Controller_6_7

fixes #66069

Why?

WordPress 6.7 is going to add support for the exact search_semantics to the REST API posts controller ( see https://core.trac.wordpress.org/changeset/59034/ ) and since the Gutenberg 19.4.0 is overriding related controller ( Gutenberg_REST_Posts_Controller_6_7 ) it's necessary to port the logic in order to preserve the new feature when Gutenberg is run on WordPress 6.7

How?

Ports the logic introduced to WordPress in the https://core.trac.wordpress.org/changeset/59034/

Testing Instructions

Create posts similar to the ones in related PHPUnit tests ( see https://core.trac.wordpress.org/changeset/59034/ ) and perform search via the REST API endpoints (again, similarly to what the WordPress PHPUnit tests do)

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