As of WordPress 4.3 the `wp_new_comment()` function has been updated to allow the comment_agent value to be set when a comment is created. The comments API endpoint now allows the comment author's user agent to be set when creating a comment.
Also, the `readonly` property on the `author_user_agent` parameter in the schema was removed.
Props rabmalin for the initial patch.
Fixes#38425.
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