MDL-76723 core_external: Handle format properties properly

- If a text format property defines the 'optional' attribute as true,
then the $required parameter that
is passed to \core\external\exporter::get_format_structure() should be
VALUE_OPTIONAL.
- If a default value is provided for a format property,
\core\external\exporter::get_format_structure() should pass the default
value to the external_format_value constructor.
- Added validation for external_format_value for the default parameter.
  Debugging will be shown in case an invalid default value is passed.
- Amended unit tests for better coverage.

lib/classes/external/exporter.php

@@ -425,10 +425,12 @@ abstract class exporter {
      * @return external_format_value
      */
     final protected static function get_format_structure($property, $definition, $required = VALUE_REQUIRED) {
+        $default = null;
         if (array_key_exists('default', $definition)) {
             $required = VALUE_DEFAULT;
+            $default = $definition['default'];
         }
-        return new external_format_value($property, $required);
+        return new external_format_value($property, $required, $default);
     }
 
     /**
@@ -547,7 +549,12 @@ abstract class exporter {
                     if (isset($returns[$formatproperty])) {
                         throw new coding_exception('The format for \'' . $property . '\' is already defined.');
                     }
-                    $returns[$formatproperty] = self::get_format_structure($property, $properties[$formatproperty]);
+                    $formatpropertydef = $properties[$formatproperty];
+                    $formatpropertyrequired = VALUE_REQUIRED;
+                    if (!empty($formatpropertydef['optional'])) {
+                        $formatpropertyrequired = VALUE_OPTIONAL;
+                    }
+                    $returns[$formatproperty] = self::get_format_structure($property, $formatpropertydef, $formatpropertyrequired);
                 }
             }
         }