In Adding Custom Endpoints, validate_callback for arguments list is described like below.

validate_callback: Used to pass a function that will be passed the value of the argument. That function should return true if the value is valid, and false if not.

source: https://github.com/WP-API/docs/blob/master/extending-the-rest-api/adding-custom-endpoints.md?plain=1#L123

But it seems that extra descriptions are required.

• It also accepts WP_Error as return value. WP_Error adds extra messages to response.
• false is checked strictly, so other falsey values like '', 0, and null will be recognized as valid.

Code is here: https://github.com/WordPress/WordPress/blob/master/wp-includes/rest-api/class-wp-rest-request.php#L911-L923