Add more missing json_encode flags (#46204)

The functions `wp_send_json()`, `wp_send_json_success()`, and
`wp_send_json_error()` also have a `$flags` argument that defaults to 0.
Given the nature of these functions (returning-and-exiting from ajax
endpoints after setting the content type header),
`JSON_UNESCAPED_SLASHES` is likely correct for all of them.

Also there was one `wp_json_encode` missed, one done incorrectly in an
unused test, and three added since the last PR.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/20037516287

Upstream-Ref: Automattic/jetpack@6516416

Author: anomiex

wp-cache.php

@@ -378,18 +378,18 @@ function wpsc_ajax_activate_boost() {
 	check_ajax_referer( 'activate-boost' );
 
 	if ( ! isset( $_POST['source'] ) ) {
-		wp_send_json_error( 'no source specified' );
+		wp_send_json_error( 'no source specified', null, JSON_UNESCAPED_SLASHES );
 	}
 
 	$source = sanitize_text_field( wp_unslash( $_POST['source'] ) );
 	$result = activate_plugin( 'jetpack-boost/jetpack-boost.php' );
 	if ( is_wp_error( $result ) ) {
-		wp_send_json_error( $result->get_error_message() );
+		wp_send_json_error( $result->get_error_message(), null, JSON_UNESCAPED_SLASHES );
 	}
 
 	wpsc_notify_migration_to_boost( $source );
 
-	wp_send_json_success();
+	wp_send_json_success( null, null, JSON_UNESCAPED_SLASHES );
 }
 add_action( 'wp_ajax_wpsc_activate_boost', 'wpsc_ajax_activate_boost' );
 
@@ -3310,7 +3310,7 @@ function clear_post_supercache( $post_id ) {
  */
 function wpsc_ajax_get_preload_status() {
 	$preload_status = wpsc_get_preload_status( true );
-	wp_send_json_success( $preload_status );
+	wp_send_json_success( $preload_status, null, JSON_UNESCAPED_SLASHES );
 }
 add_action( 'wp_ajax_wpsc_get_preload_status', 'wpsc_ajax_get_preload_status' );