Add comment_count Support to WP_REST_Posts_Controller

[patil-vipul]

Summary

This PR adds support for ordering posts by comment_count in the REST API by extending the orderby parameter in the get_collection_params method of the WP_REST_Posts_Controller class.

Changes Made

• Added comment_count to the list of allowed orderby parameters in get_collection_params.
• Ensured that posts can now be sorted by the number of comments when querying via the REST API, providing a new way to retrieve popular or highly engaged posts.

Why This is Needed:

• Enables sorting posts by comment count directly from the REST API.
• This enhancement lays the groundwork for integrating comment count sorting into Gutenberg’s Latest Posts block, addressing a feature gap mentioned in Gutenberg Issue #20046.

Trac Ticket: https://core.trac.wordpress.org/ticket/63130#ticket

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