MDL-86170: use get_custom_rules in get_available_custom_rules

The call to $this->cm->customdata in
activity_custom_completion::get_available_custom_rules can sometimes
lead to a recursion in __get() which then throws the error “Cannot
access private property cm_info::$customdata”. This should use the
direct get_custom_data method instead.

Author: rhell4

public/completion/classes/activity_custom_completion.php

@@ -101,7 +101,7 @@ public function is_available(string $rule): bool {
     public function get_available_custom_rules(): array {
         $rules = static::get_defined_custom_rules();
         $availablerules = [];
-        $customdata = (array)$this->cm->customdata;
+        $customdata = (array)$this->cm->get_custom_data();
         foreach ($rules as $rule) {
             $customrule = $customdata['customcompletionrules'][$rule] ?? false;
             if (!empty($customrule)) {