Calling update_rocket_option produces a fatal error if Cloudflare add-on connection fails: "add_settings_error is not defined"

[alfonso100]

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Describe the bug
When calling rocket_update_option programmatically, if our Cloudflare addon is active, but the connection fails (for example if fields are empty) we produce a fatal error when calling WordPress core function add_settings_error because is not defined:

wp-rocket/inc/Addon/Cloudflare/Subscriber.php

Line 632 in 7999ed2

add_settings_error( 'general', 'cloudflare_api_key_invalid', __( 'WP Rocket: ', 'rocket' ) . '</strong>' . $connection->get_error_message() . '<strong>', 'error' );

The fatal error:

Fatal error: Uncaught Error: Call to undefined function WP_Rocket\Addon\Cloudflare\add_settings_error() in /wp-content/plugins/wp-rocket/inc/Addon/Cloudflare/Subscriber.php:633 Stack trace: #0 /wp-includes/class-wp-hook.php(326): WP_Rocket\Addon\Cloudflare\Subscriber->display_settings_notice(Array, Array) #1 /wp-includes/plugin.php(205): WP_Hook->apply_filters(Array, Array) #2 /wp-includes/option.php(881): apply_filters('pre_update_opti...', Array, Array, 'wp_rocket_setti...') #3 /wp-content/plugins/wp-rocket/inc/classes/admin/class-options.php(79): update_option('wp_rocket_setti...', Array) #4 /wp-content/plugins/wp-rocket/inc/functions/options.php(52): WP_Rocket\Admin\Options->set('settings', Array) #5 /clean_prod_cats.php(11): update_rocket_option('lazyload', 1) #6 {main} thrown in /wp-content/plugins/wp-rocket/inc/Addon/Cloudflare/Subscriber.php on line 633

To Reproduce
Steps to reproduce the behavior:

1. Enable the Cloudflare add-on, and leave the fields empty
2. Create a PHP standalone snippet to update an option, for example:

<?php 
// Load WordPress.
require( 'wp-load.php' );
update_rocket_option( 'lazyload', 1 );
echo "lazyload enabled";

3. Trigger/visit the snippet
4. See error

Expected behavior
This scenario is kinda edgy, but we could check if add_settings_error is defined here to prevent the error:

		if ( is_wp_error( $connection ) ) {
			if(function_exists("add_settings_error")) {
				add_settings_error( 'general', 'cloudflare_api_key_invalid', __( 'WP Rocket: ', 'rocket' ) . '</strong>' . $connection->get_error_message() . '<strong>', 'error' );
			}
		} 

Additional context
Ticket: https://secure.helpscout.net/conversation/2685627480/508599/
Slack Thread: https://wp-media.slack.com/archives/C08N8J6VC/p1724406001657579

Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming