trying to send request-body to rest_api custom csp-endpoint

For sure, you are missing required argument permission_callback

If you want this endpoint to be public you add it like this:

'permission_callback' => '__return_true'

It is tricky, because documentation do not tell you directly that this one is required. From version 5.5 you have only debug note that this argument is required.

Also in your callback function you have param $request that will provide you details from request, no need to use WP_REST_Request like that.

/**
 * Creates parsed post based on content from Surfer.
 *
 * @param WP_REST_Request $request - request object.
 * @return WP_REST_Response
 */
public function surfer_import_post( $request ) {
    // Get Param
    $something = $request->get_param( 'something' );

    // Return response
    return new WP_REST_Response(
        array(
            'message' => __( 'Hurray!', 'your_namespace' ),
        ),
        200
    );
}